Thanks for the extremely detailed replies J-Hop2o6 and Viper Matrix Wireless. I think I mostly understood everything.
A couple points for clarification:
1) When I was asking if coverage for 1900 Mhz HSPA+ would be just as good as AWS, when the refarm is done, I meant in terms of geographical area covered (numbers of towers, I suppose?), not technically how good is the building penetration (although that's a nice plus to know about). Will I get coverage in all the same places that I might go with my AWS phone right now, with a phone that does HSPA+ on 1900 Mhz?
2) Is there a way if I can tell right now if I'm getting HSPA+ on 1900 Mhz, without have a 1900 Mhz HSPA+ phone (I have a Nexus One)?
3) Viper when you say that it would be best to get a phone that has the 850/1900 bands, are you saying that this will be preferable to a phone with AWS HSPA+? That is, if it's a choice between one or the other, since there are very few phones that have both.
4) Eventually will this mean that AT&T phones are completely compatible with T-Mobile's network both on HSPA+ and LTE (using AWS)? They would work just as well as a T-Mobile phone?
Thanks again for all the details and explanation above.