Thanks for the further explanation and links. I could be wrong, but it still seems to me like the AT&T One XL might work on T-Mobile. If you look at the specs on HTC's website, you can see that it has AWS not just for LTE, but also for WCDMA (i.e. UMTS).
www.htc.com/us/products/htconex-att#tech-specs
I don't see why that wouldn't work with T-Mobile. As nickmgray points out above, it's plausible that AT&T specified the bands for the One XL, before they knew that the T-Mobile merger was going to fall through. Hence at the time, they may have wanted a phone that would work with T-Mobile's network.
*
There's also rumors that One X will be coming to Sprint and to T-Mobile as the G4X. The time the possibility of announcement is probably in May.
Yes, I linked to the original source of that rumor above.
*
I'm not holding my breath on the spectrum refarming being a solution for the One X. Unfortunately I can't remember where I first read about it, but my understanding is that it will take a couple years. At that point there will certainly be more compelling phones than the One X.
Also, if you look at the charts in the link you provide to the T-Mobile Strategy Presentation, you can see that even after the refarming, it appears T-Mobile will still be using AWS for HSPA+, as well as for LTE. So it seems that a phone that does not have the AWS bands could suffer significant coverage issues. T-Mobile's goal in the document seems to be to make the network better for in-bound international roaming and more compatible with devices from other carriers. But a phone without AWS for a U.S. resident would still be limited.
It would be nice though if international and AT&T phones were fully compatible with T-Mobile's network. That would give T-Mobile a nice advantage with handset options; and a good way to tempt away AT&T customers (who could then keep the phones they already have). But, again, it seems like this is something for the future and not a way to count on using the One X with T-Mobile.
I'm hoping HTC will just make an international version with the AWS bands, as is the case with the Sensation. Or it would be great if they put a pentaband radio in the international version, like Nokia has been doing for a while. Or maybe the Tegra-3 One X for T-Mobile rumor will turn out to be true.
I actually like the size of the One S better. But it's not worth it to me without the 720p resolution and 16 Gb of memory (without a microsd expansion slot).