Seeing as eBuddy and Meebo either only kinda work or support only AIM, and Trillian Astra isn't actually available yet, JiveTalk looks like the only good solution for mobile IMing on the iPhone. Not only does it support AIM, there's MSN, Yahoo, GoogleTalk, ICQ, and Jabber support as well. And it's FAST.
In our own tests, the IMs came in super speedy and in iChat-esque bubbles (like the SMS app). There's even chat icons on the top to represent tabs, so you can cycle between different conversations. It's still in alpha, which explains why there aren't any options for, say, hiding offline contacts, but it's still good enough to earn our recommendation as the best chat app so far. [BeeJive]