Pre-Paid Cell Phones?

7-11 has prepaid mobile phones from Nokia and Virgin for $99 with $60 worth of mail in rebates so the total is closer to $39 – which is great if you live in the US and can wait 2 months for the refund, but not so great if you are in the states visiting for a week or two and just need something to stay in touch while you are here. Does anyone have any recommendations of places or offers where you can pick up a prepaid mobile phone for cheap?