First, download AIM onto your iPhone, then start it up. If you want to send a text message to (617) 555-1212, just send a new IM to +16175551212. You should receive a confirmation message from AOL saying your message has been sent, and the user can reply and you'll receive it on your phone as an IM. Hope this hint saves some people a few bucks.
[robg adds: We covered this technique for SMS via iChat a few years back, but I felt it worth mentioning again in the context of the iPhone.]

