however, all model are shipped as GSM unlocked so there's nothing stopping anyone from signing up with another GSM provider.
But do we know which 3G bands it supports? i.e. for American users will it work on t-mobile 3G?
I would assume so, unless T-Mobile uses a different GSM freq from international carriers. They don't want to have to make separate hardware for the international market.
Probably will work on T-Mobile's EDGE, but not GSM (they use 1700 Mhz, and the iPad spec sheet says it supports 850/1900/2100 Mhz).