The radio would have to already be able to use their frequencies since t-mobile uses different frequency then everyone else for GSM 3G.
It would however work with slow Edge speeds. The iPad is not locked to any particular carrier so its just a matter of a wireless carrier wanting to do it. when the first iPad came out I was able to put a t-mobile prepaid simcard in the ipad and recieve free edge data. They however started blocking the iPad sever months after that.
T-Mobile is slowly losing market share and is doing what they can to attract iPhone using customers, what is chances they are begging/bribing Apple to support their network with iPad 3 (let alone iPhone 5)?