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.