You can chat via Yahoo and MSN from your mobile phone.. I've tried it out and it's absolutely cool! No need to have a comp to do IM now :) - chat while traveling on a long bus trip or while waiting for a refund cheque....
My next phone I suspect will be geared to mobile internet - chat.. sports.. games.. the works/
Just hope they'll integrate GMail which shouldn't be too hard for them since it's Jabber based..
Meanwhile for Gtalk - there's MGTalk available from http://mgtalk.sourceforge.net/
I find Celtel's GPRS more reliable than Safaricom's which keeps timing out alot.
If u need more help with that i could give u some pointers...
I've tried this on a Nokia 6610i using Safaricom.