Carrier/Provider Pre-Paid Plans in the U.S.?

[5er driver]

I live in Germany. I have a factory unlocked iPhone4, and have a pre-paid plan for both phone & data and a micro sim from carrier/provider Simyo. I'll be in the U.S. for a month and of course don't want to pay any roaming charges to Simyo. Are there carriers/providers in the U.S. who offer pre-paid plans for phone & data with micro sims? I figured I'd just use one of those while I'm there. If I use a sim from AT&T will it lock my phone to them?

Thanks.