Some T-Mobile towers. Where AT&T and T-Mobile towers are close together, no doubt AT&T will keep whichever one has the best location and/or equipment.

T-Mobile 3G will continue until the merger (err..takeover) is complete and for an unspecified time thereafter. AT&T will need time to get everyone on AT&T-spec phones, probably at least 6-8 months. Actually, I'd be surprised if T-Mo USA 3G is switched off in less than 2 years. So the N900 has quite a while left with 3G and of course 2G will still work. By the time T-Mo's 3G is gone we'll all be wanting LTE units anyway.

This is mostly Very Bad News. I've been with Aerial/Voicestream/T-Mobile for a very long time. Always delivered good service at the best price. The only upsides I can see is even better coverage and eventually, a better LTE network than either AT&T or T-Mo could've delivered individually. At increased cost of course.