I have been playing with a chatboard and Arduino. Unfortunately, I never got it to work. Your experience might be different though, I think my electronics knowledge (better: lack of...) is probably the cause of that.
Here are some more sources:
http://www.brolinembedded.se/projects/keyboard/
http://www.ubiqkom.org/HOWTO/CHATBOARD/chatboard.html
https://securehost34.hrwebservices.net/~magefor/cgi-bin/view/Main/EricssonChatBoardPC
http://www.mp3car.com/vbulletin/input-devices/34000-modifying-sony-erricson-chatboard-car-pc-2.html
http://www.cs.helsinki.fi/u/jikorhon/gp32/
Please realize that there are 2 versions of the chatboard (CHA-01 and CHA-10), I can't remember the differences, but you will find them if you look for it or in the pages mentioned above.
Good luck and keep us updated!
JD