First, I strongly recommend getting a PDA or phone based on the Windows Mobile OS if using it as a dictionary is one of the main reasons for getting it. The Japanese support in Windows Mobile is far better than anything else available (as of November 2010). iOS (iPhone, iPad, iPod Touch) has only keyboard entry for Japanese--through the keyboard you can enter a kanji or word written with kanji only if you know how it's pronounced. You can use the tools in Kanjidic to look up individual kanji, but if you're doing that often or if you need to enter a word with more than one kanji, it quickly becomes a pain in the neck. So far, Android, like iOS, has only keyboard Japanese entry--but unlike Apple's keyboard system, Android's doesn't work very well (even on the phones sold in Japan).
The most important thing is to get the Japanese version of whichever PDA or SmartPhone you choose--if you buy an English-language WM device, don't expect to be able to read and write Japanese on it (it's possible to install good Japanese support on some English-OS WM devices--but it's a bit different for every device and takes a bit of hacking; you can also convert some English-OS devices to the Japanese version of WM, but doing so yourself entails some difficulty and risk and is not possible for all models--see here for more info and instructions for some Dell Axim models you may be able to find used for a good price). You should also think about the display resolution: the standard 240x320 QVGA displays are OK and compare well to standard electronic dictionaries, but the dictionaries (and everything else) will look much better on a 480x640 full VGA display; if you want to do more with your phone or PDA, especially surfing the web or using GPS navigation, VGA or better is a MUST. Size also counts: on anything with a 3.3" or smaller VGA or QVGA display or a 4" or smaller WVGA display, kanji can be hard to read and using the onscreen keyboard and handwriting entry is difficult. On used PDAs and phones, avoid Windows Mobile 5.0--users are reporting instability and bugginess; WM 2003, 2003SE, and WM6 are reliable, rock solid on most machines, and just as capable, so stick with those; no verdict yet on WM7, which should start showing up on new phones in late 2010. And, finally, you need a big memory card--a full slate of dictionaries can take well over 1 GB, so you should really pick up a 4 GB or larger card so you'll have room to do other things with your PDA, too (music, movies, GPS navigation, etc.). For making your own system, the Dell Axim X50V with WM2003 OS is probably the best buy, as the English version can be converted to Japanese, it's got a great VGA display, it's a good size to carry around, and dual memory slots means you can pack up to 68 GB of memory cards in it at once. DO NOT get an X51V unless you're fairly skilled at hacking operating systems--it comes with WM5.0, as mentioned above, and users have been reporting all sorts of problems. While it runs beautifully if updated to WM6 and hacked to add Japanese, doing so is complicated and requires some experience in rewriting operating system code (none of the off-the-shelf WM6 upgrades work perfectly until you've gone and tweaked the code a bit; and some aren't compatible with the hacking necessary to add Japanese). Click here for a fuller discussion. If you haven't bought a PDA yet, take a look at my offerings--an X51V working beautifully, fully Japanese-capable English OS, with all dictionary software and dictionaries already installed and tested; and a Dell Axim X30 with the Japanese version of the WM2003SE OS, all set up and ready to use with the same dictionaries, dictionary software, and Japanese entry as the more expensive X51V version (all JLT PDAs have been factory refurbished to new condition and still, as of fall 2010, are as good as the best current systems in performance and capability).
First, install a dictionary reader program. I've tried and rejected almost every WM program out there. Most are terrible, and most of the ones that aren't bad work only with expensive but mediocre dictionaries sold by the program maker. The best program I've found is EBPocket. Because the large commercial Japanese dictionaries come in or can easily be converted to the EPWING format EBPocket uses and the program itself is excellent, that's where I've put my efforts. Here are instructions for setting up EBPocket, and here for using it. Don't actually start EBPocket for the first time until AFTER you've put the dictionaries you want on your memory card.
Now, you need to get some dictionaries. The JLT dictionaries form a pretty comprehensive set (click "screenshots & details" next to each for a fuller description of each dictionary; all except Eijiro and Conjugations, which are available from JLT in the JLT package, are free). Most of these are based on the Edict project from Monash University and Prof. Jim Breen, extensively modified and converted to EPWING format by me. There are two Kanjidic options--choose one or the other, not both.
(click here if you have trouble downloading; click here if you'd like to buy a memory card with these files already installed). Very important: Each EPWING dictionary is an entire folder with various files and subfolders inside. After unzipping the download, leave everything in the dictionary folder alone--moving or renaming anything or trying to open a file directly with any program will break the dictionary. When copying or moving a dictionary, be sure to copy or move the entire outermost dictionary folder--again, don't mess with anything inside of it and you'll be fine. Now, when you start up EBPocket for the first time, it'll automatically find and set itself up to use all the EPWING dictionaries anywhere on your device. If you've already been running EBPocket and want to add another dictionary, use the obvious "Add Dict" function or, for more control, "Edit Group" from the Tool menu.
You can also buy EPWING versions of most of the major commercial dictionaries on CD. Perhaps more conveniently, from Logovista, you can download some of them in Logovista's own proprietary format, which can be converted to EPWING quite easily using a program called dessed (how to navigate Logovista's site to get to the dictionaries; how to use dessed). If you don't get Eijiro, a particularly good buy is the Kenkyusha Intermediate E>J and J>E dictionary (if you already have Edict and Eijiro, though, the Kenkyusha Intermediate doesn't add much). The Kenkyusha Daijiten is much larger and far more useful, but at 25,000 yen it's quite expensive. Very useful, at least for high-level students, is a good kokugo jiten like Koujien or Daijirin. There are also good commercial French<>J, German<>J, Italian<>J, and probably many more dictionaries available in EPWING or Logovista format.
Finally, there are all sorts of EPWING dictionaries available for free online. Some of them are versions of the same dictionaries I've used (Edict, Enamdict, Kanjidic)--I think mine are quite a bit better, obviously (if I'd thought what was already available was great, I wouldn't have spent months making my own). However, some of the other choices are interesting. Note that these aren't my work, I haven't even tried some of them, and therefore I can't make any promises about them. Maximilk's site is a good place to start. See the "other dictionaries" page for descriptions and links.Sanskrit-EnglishS
Shrink your dictionaries. EPWING format dictionaries are huge memory hogs--use a program called EBShrink to shrink the dictionaries into the much smaller .ebz format, which works just fine in EBPocket (shrunken dictionaries have the full contents and run just as fast as unshrunken ones). EBShrink comes as a free helper file included in the EBWin package, which you can find about halfway down the very long EBPocket page (install the unicode version). EBShrink is pretty self-explanatory, but click here if you'd like instructions. Note that my dictionaries are all pre-shrunk and ready to install--DON'T try to shrink them again!
Install your Dictionaries. First, if installing to an SD card, you must format the card. The best way is in the PDA itself using the CNetX FlashFormat program, but a digital camera will probably do in a pinch. If you don't format it, or format it in a Windows or Mac computer, it'll seem to work for a while, but then your files may start disappearing from the PDA. Each dictionary is an entire folder--don't rename, move, or try to directly open anything inside it--just put the entire folder anywhere on your memory card (except My Documents), then open EBPocket and access the dictionary from within the program. If you install the dictionaries to the card before starting EBPocket for the first time, it should find them automatically. To add dictionaries later, use the "Edit Group" function in the "Tools" menu.
To make your own EPWING dictionaries, click here.
And the Sharp Zaurus? I don't know much about it, but I do know the Japanese versions have kanji handwriting recognition built in, and that it's supposed to work very well. Also, the Zaurus has an dictionary program built in, so just put my EPWING dictionaries on a card and stick it in and you're good to go. There are some resources listed on Jim Breen's Japanese page. In addition, the PDIC .dic format files for Eijiro and the files I made for WDIC might run on ZPDview for the Zaurus (again, whether or not ZPDview works with recent Zaurii is unknown). There may be other programs that can run PDIC files as well--it's worth looking into. Armin Rump's excellent site (which provided the key information I needed to make my Edict files work with WDIC) has clear and useful information for using Japanese dictionaries on the Zaurus.