The AIM iPhone app has never been great, but it just got some pretty solid features—it pings when you have new IMs, even after you quit the app, and lets you send free SMS messages.
No, it doesn't use Apple's long lost push notifications. Instead, it uses SMS notifications, for up to 24 hours after you sign out of the iPhone app (you can pick shorter intervals, as well as limit to pinging you only when you get IMs from new buddies, not the people you were just talking to). Hopefully, they'll get a bit more sophisticated, since right now they just tell you to fire up AIM 'cause you got mail.
Other notable new stuff—free SMS texts to anyone in iPhone contact list, multiple account support and location awareness, so you'll see buddies in a special new group called "near me." (You can limit this to buddies, everyone or no one.) It seems to be a little bit more responsive than I remember, too.
Also, AIM is splitting into a paid and a free ad-supported version. So far, I've just seen occasional pop-up on the bottom of my buddy list, but nothing while actually IMing, so not too annoying. If you hated AIM before, you're not gonna like it now, but it's definitely better. [iTunes, Hat tip Ars Technica]