Google still hasn’t come out with their mobile version of GTalk. But there’s a J2ME client out there for Google Talk called GoTalk Mobile.
This can be used to sign into your GTalk account from your J2ME enabled device using GPRS. It sends messages similar to SMS but uses GPRS instead. The only cost that you would have to bear with is the GPRS Bandwidth usage charges which would definitely turn out to be more economical if you are "On The Move" constantly.
Their latest version is not very PDA friendly but the older version works quite well. If you do not own a PDA, then download their latest version which has quite a snazzy feel to it.
I hope the think tanks in Google are reading this!! Still waiting for GTalk Mobile……