I just came back from US, and on that trip, I bought a H2O wireless SIM and $20 worth of time/text ($.14/min. and $.10 per text out/$.05 in). Worked fine in my iPhone (unlocked).
Kinda PO'd, though, that their prices to Canada went from $.28 (back when they were O2 Wireless) to $.49/min.
But setup and activation were pretty simple and straightforward. I think the minutes are good for 90 days.