Over on Reddit a Nokia Lumia 928 owner is reporting this his Windows Phone is GSM unlocked and delivers HSDPA on both AT&T and T-Mobile.
Volvoshine, a well known figure in the community, writes:
Verizon Nokia Lumia 928 is Unlocked out of the box
Got a chance to try it today..With an AT&T and T-Mobile micro sim.. After using the "Access point" app to apply the correct APN’s… it worked flawlessly (On HSPA 4G, not LTE)
The Nokia Lumia 928 is a world phone, with both CDMA and GSM radios, and it appears it supports the frequencies needed for HSDPA on the two major GSM networks in North America without needing to call Verizon.
Of course LTE does not work, meaning it is still a better idea to buy a Nokia Lumia 920 or 925 directly, but this find does give Lumia 928 owners a lot more flexibility in their use of the handset.