After reading most of the comments I think that AT&T will keep the T-Mobile bands and upgrade them throughout the next couple years. What I think sounds like a good plan:
Keep all the bands. This will expand the bandwidth/usage more evenly.
Most phones support EDGE so that would mean that when current AT&T users leave a covered area by AT&T 3G & EDGE and happen to be on a T-Mobile covered area they can still get service. The same applies for T-Mobile users. This means more coverage!!!
Eventually (hopefully sooner than later) require that phone manufactures support all the bands or at least EDGE. AT&T can upgrade each cell later to support faster speeds.
Your prices won't go up, AT&T still has to compete with Verizon so just because they are basically the only carrier to have SIM cards they can't pull prices up. Also your current plan won't go up. I have been with AT&T for about 7 years but, at first we where with Cingular; our rate plan no longer exists but we still pay the same thing, you've seen it with Data Grandfathering on the iPhone the same applies to your rate plan.
In my opinion this will be a boost to customers coverage area and your plan won't change either. The only bad thing I see is the job losses from people working at T-Mobile especially at corporate where you'll see that there will no longer be a need for most jobs as AT&T already has people employed to handle then.
should make them do it as its 1 of the selling points of the iphone so if the new iphone comes out with something new you can bet top dollar that the UK phone carriers will not have it activated
