Thanks @edcoolio !
Short answer, I don't have any data yet. The hardest part for me so far in tracking this stuff down is knowing which carriers use which LTE frequencies in which areas here. I've been digging to find T-Mobile USA data for myself and some of my friends/co-workers. Here's what I know:
T-Mobile USA historically used 1700/2100 MHz for their LTE. This is the existing T-Mobile USA LTE infrastructure. This is sometimes just called "1700" for short , is commonly called "AWS", and also known as "LTE Band 4".
T-Mobile USA has been increasingly reframing its 2G/EDGE network to work with LTE. That is supposed to be going on right now and substantially increasing. First the reframed to HSPA+ at 1900MHz. Now I believe this is moving to LTE since that's what everyone wants. This LTE is sometimes called "1900", is commonly called "PCS", and is also known as "LTE Band 2".
T-Mobile USA also purchased a lot of 700 MHz A Block spectrum from Verizon, which they have already rolled out in some markets. Note that this 700 MHz A Block is often called "700 A" and also known as "LTE Band 12".
More fine print: There are different bands within the same frequencies in the LTE space. Just because a phone has the right frequency doesn't necessarily mean it has the right band.
In short, if you're looking to be on T-Mobile USA 4G LTE you're going to want to look primarily for LTE Bands 4, 2, and 12. In that order.
All that said, I'm one of the few people who's just really happy with being on HSPA+, so the whole LTE push hasn't bothered me at all.
Sorry for too much writing. Hope it's helpful.
Addendum. Here is a map showing the presence of LTE Band 12 in the USA.
http://www.tmonews.com/700mhz-lte-map/