I don't normally go and make assumptions like this, but after playing around with the newly installed 3.0 beta 2 yesterday, I noticed something that leads me to believe that a native iChat/AIM client is coming in future builds on 3.0. Why? If you look at your contact lists page now, it lists and gives you the option to add an AIM screename onto their card. As of now, nothing happens when you tap on their screename, but in the future, I would assume it would open up the iChat client. I still believe it will be integrated into the messages app in some fashion, so your SMS, MMS, and IM messages will all be in one place.