What's the best prepaid plan for someone who needs a temporary number but doesn't want to buy a new phone?

I’ll be in California (LA, SF) for a few weeks. I don’t want to use my current phone number while I’m there, but I do want to use my current phone—an unlocked, quad-band GSM phone that takes a sim card.

I’d like to find a mobile phone plan that offers a prepaid, re-loadable sim card that I can use while I’m there, and which I can simply discard afterwards.

The prepaid plans from AT&T and T-mobile seem to assume that you’ll be buying a phone from the carrier together with the phone service, which is more expensive and doesn’t match what I’m looking for.

Any suggestions?