The good folks at Engadget have been doing some digging, and have managed to confirm that the original AT&T/Apple exclusivity deal was set to run five years after it was first entered in 2007, which would put it in place until 2012.
The way they found this out, was due to a California class action suit, which alleged that
“Apple and AT&T illegally exerted a monopoly over iPhone service by telling customers the iPhone’s required service contract was two years long when the Apple / AT&T exclusivity deal was actually for five years — thus requiring buyers to re-up with AT&T for three years (and not, say, T-Mobile) if they wanted to keep using the iPhone”
AT&T had to respond to this, and in doing so confirmed the five year agreement.
It’s unknown if this deal is still present, or if the agreement between the two companies has been re-negotiated, altered or breached. Just don’t be surprised if you’re stuck with Ma Bell for two more years yet.