First off, this is my first post and apologies if it is a repost.
I just checked out T-Mobiles website and noticed that they have a full fledged IM client available for download for all 4 major IM services AIM, Yahoo!, MSN, and ICQ.
It is available for Download here
There is a disclaimer from T_Mobile Note: The Instant Messaging application for the RIM 7290 will NOT work properly if loaded onto other RIM devices.
I installed it on my 7230 and it works perfectly. Not sure about any other models or on other carriers handhelds. I do know that mine is working A'OK.