I have AIM working on T-Mobile with unlocked 8800. I'm honestly not too impressed with it. It puts the icon in the Instant Messaging folder making it a pain to get to. Then when you receive a message, if you're not in the app, you have to go back into that folder to open it up. It does not list the message in your messages folder, and give you a pop-up asking if you want to view the message.

I honestly think JiveTalk is better so far.