It depends, currently even the factore unlocked 920 wouldn't work on T-Mobile as it doesn't support its AWS bands.
I doubt that we'll see a T-Mo 920 (910?) right after the 920 launches on AT&T, it will probably remain an exclusive for at least 6 months before it makes its way to other carriers. If Nokia was smart, they'd release a T-Mo variant and a VZ variant, as right now they need to focus less on getting themselves as an AT&T "premium" brand, and more so on just gaining back their North American market share.
Here's hoping for a pentaband 920, I'd love to use it on WIND here in Canada.