Well, I guess this is the best way for AT&T to get all those unlocked iPhones on T-Mobile USA... 
I would expect they'd let us grandfather our voice and text plans. Our old unlimited EDGE data plans on our unlocked iPhones for $6/month - I highly doubt.
AT&T has a rule that if they see an iPhone on their network without a proper iPhone data plan, they will automatically switch you to one...
I foresee this raising prices for T-mo customers *eventually*, but those old plans will be grandfathered for a while. They'll have to select a new plan when they upgrade to a new handset, most likely. I don't expect it will make one whit of difference to At&t customers' prices. Signal might improve a whole lot on both networks though!
I would expect they'd let us grandfather our voice and text plans. Our old unlimited EDGE data plans on our unlocked iPhones for $6/month - I highly doubt.
AT&T has a rule that if they see an iPhone on their network without a proper iPhone data plan, they will automatically switch you to one...