[Motel75]

Don't worry about eBay. Prepaid telephony is one of the great inventions of our time, and it's a mystery to me why it's used so little in the US. All it means is that you pay for the calls ahead of time, rather than afterwards. Because of this, there's no need for a contract or a credit check, and you can easily control spending. There are many ways to top up, but the most common way is to buy a voucher (available just about everywhere). You call a free number on your phone and enter the number on the voucher, and the credit is added to your account.

You have a GSM phone, but be aware that the frequencies in Europe (900 and 1800 MHz) are different from those in the USA (850 and 1900) and if your phone isn't quad- or tri-band it will not work; if it only supports one European frequency it might not work terribly well. Fortunately, you can often get a very cheap phone with your SIM card (perhaps less than 20 pounds), so this might not be such a big deal.

You should look into Virgin Mobile. They have good service, reasonably fair prices (and sometimes cheap phones) and calls to the USA cost 20 p a minute, far less than on O2 or Vodafone; also, you can keep the account active by as little as sending a single SMS per year, if you plan to return to the UK (although topping up Virgin outside the UK can be a pain, though you can buy the topup cards cheap on eBay and call international customer service to do it). If it turns out you don't need a phone, the SIMs currently cost 5 pounds with 5 pounds credit.

Orange international prices are also more reasonable than Voda, TMO, and O2. Carphone Warehouse (on every high street in the UK) has a SIM-only offer called mobileworld with the best international prices, though unlike most UK prepaid offers there is no international roaming and the card expires after a few months.