I spent the weekend mostly on the go out on Long Island using my iPhone 3GS as a laptop substitute on Saturday and as a connected camcorder at a wedding on Sunday. During that time, I also tested two multi-protocol chat applications for the iPhone / iPod touch that also support push notifications: IM+ and BeejiveIM. I wrote up a review today that is now up on Obsessable for your perusal. The short form of it is that both work well in terms of push notifications and handling multiple chats and multiple IM platforms, although IM+ adds Twitter and Skype support that BeejiveIM doesn’t have, and BeejiveIM has a slightly nicer overall look and feel and the ability to save transcripts of your chat conversations by emailing them to yourself. Both apps are buggy under the right circumstances and BeejiveIM becomes very stubbornly and frustratingly uncooperative if you happen to log in to AIM via both it and another application at the same time.
If you only ever live in AIM and you don’t need to worry about chat transcripts, then I’d just stick with the free AIM client and hope they come out with landscape mode soon.