This MacOSXHint explains how to send SMS messages to mobile phones from the latest version of iChat AV:
To utilize this cool feature in iChat, simply type Shift-Command-N for a "New Chat with Person," and enter the phone number in the above [+16175551212] format. When you send the message, you'll receive confirmation from AOL that it was sent. 
Update: only works in the USA :(
