We bought a Cingular pre paid package a year or so ago in the States. Cost $79 and came with $30 worth of calls.

You can top up your airtime as you need it, and they also have rollover minutes. As long as you keep topping up your account every 90 days, your minutes are stored until you next use the phone. We top up from the UK and then when we arrive, we switch on our cell phone and know that we have loads of prepaid airtime.