I can use iChat to send SMS messages to my phone, and my phone can send an SMS message back to my iChat.
First, under File>New Chat With Person, I added my phone number with a leading +, 1, the area code, and the seven digit number (so, it looks like +1xxxxxxxxxx). I can’t immediately chat with my phone because of my privacy settings, but I can add my phone as a buddy. Once I do that, I type in a message and it shows up on my almost immediately as a text message on my phone.
If I reply to the message from my phone, the message shows up in the iChat window just like any other chat session.
This is more than just the standard geekery though: this means I can easily send a text message to my wife without dealing with my phone. There were probably other ways to do that easily (such as T-Mobile’s web interface and also email), but I like how iChat adds my buddy name in front of the message rather than some odd gateway name, and that I get the reply back in iChat. Very handy.