I've installed gjiten without a problem using apt from command line, not the Application Manager GUI.

Here is a small howto for those who don't know what I meant above.
Intro:
Apt is a Debian package management system and os2008 is of course Debian based. The Application Manager is a front end to apt. Apt does all the installation but in difference to the graphical counterpart it doesn't complain as much.

Here it goes:
1. Check that you have the repository.
Open Application Manager. In the menu go Tools -> Application catalogue.
If you attempted to intall it earlier than you should find gjiten catalogue in the list,
else add it manually. It should contain:
Name: gjiten (you can choose any)
Web address: http://gjiten.garage.maemo.org
Distribution: chinook
Components: user
Close and let it refresh if it asks too.

2. Install becomeroot package http://eko.one.pl/maemo/dists/bora/u....1-2_armel.deb
using the aplication manager.

3. Close aplication manager and open xterm (in Utilities menu).
Become root with:
sudo gainroot

Install gjiten with:
apt-get install gjiten

After executing this command you need to agree with all questions by entering 'y'.

If that succeeded then you have gjiten installed and should find it in the menu with other apps.

A couple hints on using apt.
To search for packages use command:
apt-cache search name
where name is what you look for.
To refresh to package list:
apt-get update

I hope that helps.

Question: You have CJK support installed correct? if so, are you using the matchbox keyboard?

Is there anyone to install this and have it working without installing CJK support? I guess that you would need kanjipad too.

Ok, I got it installed but the characters did not show. I simply installed the Kochi-gothic TrueType font and it was god to go. And to think, I previously installed CJK support just to get this one application (Gjiten).

Thank you, cool_guy!!!

Gjiten doesn't need fonts or CJK to function, but as any other app you need fonts.
If it doesn't display characters you can get the fonts them by adding cjk catalog and installing just the fonts.
You can copy and paste japanese into gjiten, so you can use anything that can c&p, e.g. websites. There are also web based IM. If you want to type in directly you need CJK. It cannot use original on screen keyboarb therefore:
For n800 you need to install matchbox-keyboard!
N810 has an hw keyboard, so it doesn't install it by default.

It probably isn't legal, but I just grabbed the msmincho font from a Windows box and installed it on my NIT. Now Japanese shows up fine on web pages and Kiten works (KDE app). I think the KDE virtual keyboard would work for Japanese input, but it defaults to your active language and English is the only available language in the current KDE release...

what is the kde virtual keyboard? Is it usable with the maemo dektop (does it pop/float)? Is there a package for it?
That is a major issue with QT on n800!

@cool_guy

nice . i posted an extremely long way to do this...

I plan on trying it out when I clone my os again.

I have an N800 and I tried the Matchbox keyboard/SCIM and hated it with a passion, this is not an option for me. I plan to upgrade to the N810 or N830 (depending on price) soon, so this is all moot.