They have a multi-year exclusive with them. That means they can *NOT* until that contract expires. I have heard its any wheres from 2 year to 5 year agreement. Keep in mind, that only counts for the US.
Don't count on seeing it with other providers anytime soon or near future.
In about 18 months Apple will rename the iPhone to something else, meaning the AT&T contract will no longer hold. Like the way it bumped the Mac OS version number to get out of the Mac OEM builder contracts.