I was talking to my brother about iChat and instant messaging in general when the subject of text messages came up. MSN Messenger has had a feature like this but there was, to my knowledge, no comparable feature in AIM or iChat.
Well, maybe. This article points out a feature recently added by AOL that allows you to type in the phone number of a friend that has an SMS-capable phone and send them a text message.
I have not received any confirmation that my message was sent or that it works, but so far, so good.