GSM subscriptions in USA?

Having recently seen a couple not-to-badly-priced GSM/GPRS boards for arduino, I then wondered if any of these would actually be allowed onto one of the networks in the USA? Anyone actually accomplished this?

I found on the Best Buy web store that they sell Prepaid SIM cards from various networks. Is there more to it than I write some code to drive a GSM shield with an arduino board, plug in one of these SIM cars, et voila, I have wireless operation available to me? Or is it really that simple to do?

I just hate the thought of spending close to $100 for a SIM and the shield only to find out I'm either forbidden to connect or that there's endless red tape necessary to get approved for use.