Over on the Internet Brands Developer Blog, there is an article arguing that Apple's use of the Address Book to aggregate all the different ways to reach someone will eventually lead to a VoIP end-game for the iPhone:
Picture unlimited free calls over WiFi/IP without even having to "think about it," by simply picking a Person from your Address Book, and hitting "call" the same way you'd make a normal phone call.
The interesting thing here is that on a Mac, this is already possible. If you have a Bluetooth connection to a cell phone, have Skype, Gizmo, or the JAJAH plugin installed, you can use those services to call any number in your address list by right-clicking on it.
Apple, SIP, VoIP, Address Book, iPhone, AT&T, Articles