Aside from built-in Facebook integration, another major feature coming to OS X 10.8.2 is iMessage phone number merging. This functionality will allow you to receive any iMessages sent to your iPhone phone number on any approved iOS devices or Mac of your choice. In other words, if someone has your phone number and sends you an iMessage to that number (rather than your Apple ID email address,) Mountain Lion 10.8.2 will be able to receive and display them in the Messages app, and you'll be able to respond to them from there as well. It's the final missing piece in making iMessage device agnostic (well, as long as it's an Apple device.)
To merge your phone number with your Apple ID after you update to OS X 10.8.2, simply launch Messages and log in to iMessage. You'll then get a prompt that asks if you'd like to associate your phone number with your Apple ID. If so, just select Yes, and you're done! OS X 10.8.2 is set to be released this fall.