Click the BlueTooth button in the toolbar of AddressBook to connect to your cell phone, then control- (or right-) click on a mobile phone number to access the SMS messaging dialog. You can SMS anybody with a mobile phone right from your mac.
Also, while you're connected to your BlueTooth phone, any message that comes to your phone appears on your desktop. Miss a call? A message pops up on your screen!
[robg adds: If you don't have a Bluetooth phone, you can still send SMS via iChat, as described in this hint.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031027154029634