T-Mobile, "Uncarrier"

i really enjoy what T-Mobile is doing. I'm a bit confused on something though. Do I need one of their "uncarrier" plans to go along with the phone? I find myself in a WiFi area nearly always. I find that I don't really need service as of now. Can I just go in, give them $99 down and $X amount for the phone each month, or must it be accompanied by their $50 "plan"?

Sorry for the somewhat stupid question.