According to T-Mobile customer service, unless you are using an unlocked GSM iPhone you may not get any data service at all. Verizon and Sprint's iPhones both are locked out of GSM voice and data. Sprint iPhones can roam on T-Mobile's data but only if activated on Sprint's network with active account on Sprint; Verizon iPhones will work for 911 calls only. Also, Verizon and Sprint iPhones won't work with a T-Mobile or AT&T SIM cards, only with CDMA SIM cards provided by Sprint or Verizon. In addition, be advised that your Sprint/Verizon iPhone will show up on T-Mobile's network as a non-T-Mobile phone and you will probably be charged roaming fees. T-Mobile customer Service also told me that since you phone was not activated with the CDMA carrier it was made for (Sprint or Verizon), your phone could also show up on T-Mobile's network as stolen, at which point the phone won't work on anyone's network, and you could face legal problems stemming from that.
Be advised, that sometime this year the iPhone is coming to T-Mobile, and there are strict requirements from Apple that they must abide by to get those phones, why not wait till they get it before getting yourself in real trouble. Especially since your T-Mobile SIM card won't work ona Verizon or Sprint iPhone to start with?
Oh, and your Sprint/Verizon iPhone will require the Sprint/Verizon sim card that came with when first turned on. It will report itself as stolen if another company's SIM card is put in it when first turned on. This was background update done by Apple onJanuary 2 this year.