The race to see who will rule IM on the iPhone has begun. Last week, Dave wrote about Meebo. It works, but has flaws (the buddy list is very far from the chat window for some reason, and the chat window can't be resized).
Today, I came across JiveTalk, which offers simultaneous login to AIM, MSN, Yahoo!, GoogleTalk, ICQ, and Jabber. The alpha release for iPhone worked pretty well during my brief testing.
The UI resembles iChat. I had multiple AIM conversations open, and a small notification window told me when the inactive conversation had received an update. Also, two "chat bubbles" at the top of the screen let me toggle between conversations.
It's alpha, as I said, but looks good so far. Check it out.