The recent behavior of AT&T regarding the upgrade from iPhone 3G to 3GS may shed light on how long we have to wait for a second iPhone carrier in the U.S.
No one outside of AT&T and Apple know for sure the duration of AT&T's exclusivity. In August of last year USAToday said the contract lasts until 2010 .
My thought is that if the exclusive contract ran out at the end of 2009 or by June of 2010 then AT&T would be behaving differently right now. Instead of telling people whoose contracts expire in a year to get lost for a year they would be rolling out the red carpet to extend their contract for another 2 or 3 years.
Eventually the exclusive deal will end and an early sign of that day will be AT&T offering current customers a deal to extend their contract. AT&T's current behavior signals that the exclusivity is here for at least a couple more years.