Yes. You will need an either unlocked or jailbroken iPhone with a UMTS radio (no Verizon iPhone4).
You will only get GSM data speed from the phone
You will need to manually enter the SMS/MMS/data servers on the phone to have full service.
If you are an old school T-mobile user like me, you can enjoy 5GB of data before they throttle you to GSM speeds.... but since you are *already* at GSM speeds, its just like having completely unlimited data.