Readers have sent this in so many times that it must be cool. You can send SMS messages from AIM (works in iChat as well, let us know about other clients). All you do is send an IM to +1FULLNUMBER. For instance, if your phone number is 813-555-3218 you send an IM to +18135553218. It comes wrapped as an SMS complete with the screen name it came from. Neat!
If you add phone numbers to your buddy list, they do not show up as online. Also, with some carriers you can reply. We've gotten confirmation that you can on AT&T, but you cannot on T-Mobile. Let us know about your carrier via a comment.
It is also not clear if this works on all carriers. There is a story today that Verizon has agreed to play along... But until the story came out we did not know it was a carrier specific thing. Again, if you have any info on this please comment.
Also, for our beloved international readers... Does this work in your country? Do you have to add a country code? Let us know. This has been pretty much an underground feature, and we'd love to fill everyone in on the ins and outs.