I'm not but i would. (This is not considering the fact that i would never buy a Iphone or us AT&T)
But the Iphone is in a sense getting 'out dated' You know they will release a new one in not to long with all the new phones like the nexus one. All these new phones carrying 1Ghz processors are much faster and can multi-task. You know if they want to stay in the game Apple will release a new Iphone within the year is my guess. So i would take it and get a new phone before they replace the Iphone.
If i didn't want to sue T-mobile i would still take it reject the 2yr plan pay $100 to now have a $450 unlocked phone and take it to AT&T. If i was a Iphone lover i would not even use it and sell it to make $400 to buy your new Iphone when they release it.