I understand that T-Mobile offers pre-pay GSM in the US. Looking at their website it seems you need to buy a phone to get the SIM, however.

I'm flying into JFK tomorrow, and would like to pick up a SIM I can use in my triband phone thereby avoiding massive roaming charges. There seems to be a lot of reference information archived here about Americans buying European SIMs but not much about the converse!

Any options at the airport, or suggestions of places where I might pick one up?