There's a leaked version of iTunes 7.7 which is for Apple and phone stores only. It lets you lock the iPhone to a particular carrier apparently. So you could lock the iPhone to Three, or not lock it at all is what I've read.
now contemplating one... eek did eye say that?! no no! i saw the new apps on version 2.0 software on the iPod touch which is also available on the 3G iPhone it looks soo purdy!