A couple of days ago I read in this news report that T-Mobile USA has acquired some lower A-Block 700 MHz spectrum from Verizon it intends to use for LTE in the near future. To me as an outsider, US spectrum trading just leaves me baffled as network operators keep exchanging spectrum for spectrum and/or money. To me it looks a bit like the 'shell game' (minus the fraud part of course…).
Anyway, last year I did an analysis of how the 700 MHz band is split up in the US to get a better picture and you can find the details here. According to my hand drawn image and the more professionally done drawings in one of the articles that are linked in the post there's only 2×5 MHz left in the lower A-band, the famously unused lower part of band 12. Again the articles linked in my post last year give some more insight into why band 12 overlaps with band 17. Also, this Wikipedia article lists the company for band 12/17.
So these days it looks like the technical issues of not using the lower 5 MHz of band 12 have disappeared. How interesting and it will be even more interesting to see if AT&T will at some point support band 12 in their devices or whether they will stick to the band 17 subset. Supporting band 12 would of course mean that a device could be used in both T-Mobile US's and AT&T's LTE networks, which is how it is done in the rest of the world on other frequency bands for ages. But then, the US is different and I wonder if AT&T has the power to continue to push band 17.
One more thought: 2×5 MHz is quite a narrow channel (if that is what they bought, I'm not quite sure…), typically 2×10 MHz (in the US, Europe and Asia) or 2×20 MHz (mainly in Europe) channels are used for LTE today so I wouldn't expect speed miracles on such a channel.