>the iPhone was AT&T exclusive for a long time at the beginning.

I think that was a very different time though. Smartphones were just becoming popular. A lot of other upcoming smartphones also had carrier exclusives at that time (Verizon with the Droid line). I don't know if that would be acceptable in today's world.

A joint move like that by the carriers would be subject to a lot of antitrust scrutiny, where as apple can move on it's own with a lot less scrutiny.