T-Mobile is refarming their network to support data on the 1900mhz band by the end of this year. There are scattered reports of people seeing it already, but they're rolling it out nationwide by the end of this year and beginning of next.
T-Mobile is readying support documents for iPhone, as an example, since I imagine they will be marketing this to iPhone users currently on T-Mobile who want 3G speeds, as well as iPhone users disgruntled with AT&T.
Fortunately, this will apply to ALL phones using 1900mhz - meaning any phone unlocked from AT&T will get full HSPA+ speeds from T-Mobile, including the 920
T-Mobile will probably natively get the 820, but I doubt they'll actually get the 920. So buy it unlocked direct from Nokia or anywhere else (not ideal price wise for people who only buy phones using their upgrades, but it's the best we've got)