T-Mobile, Cingular, most AT&T (though not the prepaid, i think), Nextel, & a few regional carriers use GSM here. I'm not sure how it works over there, but I think to do prepaid here, even with a SIM, you'll need to purchase the SIM, activate a prepaid acct with the SIM, then purchase minutes to put on the prepaid acct.
I don't think I've ever seen just a SIM card that you could purchase off the rack that was already activated with minutes on it.