AT&T and T-mobile use different bandwidths for their data connections. I think their voice runs on the same. That's why you can hack an iphone to run on t-mobile, and it works great on voice communication, but can only get edge speeds for data. Still works fine for me though, I only use to to stream pandora in the car... otherwise I just use wifi.
Once AT&T comes in and begins the pillaging, they will hit me for their SUCKY iphone plan, and I will go from spending $100 a month for both my wifes' and my phone, to $200 or so.. and I will begin looking for another carrier.
Supposedly, nothing will change for a year, then they will begin to screw people. I feel bad for the people who just plopped down a big chunk of money for the latest and greatest t-mobile android phone that will likely not work on the new network once it's said and done.