For anybody missing the ability of Address Book.app to send SMS messages (Apple Kbase article), which was present in 10.4 but removed in 10.5 and has been missing ever since, there is now a free plugin I've written to provide some of the missing functionality.
It adapts the SMS sending code from the emitSMS Dashboard widget into an Address Book action plug-in, which should be compatible with 10.5 and 10.6. Unfortunately, SMS receiving and dialing numbers via Bluetooth aren't supported, so it doesn't replace everything Apple removed in the Leopard version of Address Book, but it should provide a little more ease of use.
Just download the plug-in (96KB), expand the zip file, and then drop it in /Library/Address Book Plug-Ins. You can find more instructions and usage notes on my site.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091002073002415