BTW, does anybody know, if an unlocked HSPA+ capable device works on T-Mobile's 3G bands, would it also be compatible with their HSPA+ 4G?
Yes, HSDPA categories 7 through 10 are cross compatible (if the device carriers the bands for both carriers) hence same limitation would apply.
However, currently only Qualcomm's chipset is able to pull from T-mobile's 42Mbps network. It's difficult to speculate whether T-Mobile will get it's own handset as opposed to "universal" GSM phone.
Brian Klug hinted that the T-Mobile SGS2 may have bands for both carriers. He couldn't investigate further because he would have had to unlock the review unit. Users are reporting that the unlocked HTC Amaze operates on AT&T's network. Next gen chipsets will expand upon cross carrier compatibility.
As for the G Nexus though, if I had to speculate I would guess that T-Mobile will get their own unit rather than a cross compatible one.