Prepaid plans in the US?

I’m going to the US for a bit, and would like a local San Francisco number for my cell, so people can call me without paying an arm and a leg. (When I lived in the states and didn’t have a credit rating, AT&T wanted $800 in deposit to open up international calls on my cell!)

And I already have a quad band phone that I’m reasonably happy with, thank you very much, so I’m not interested in a new phone.

All I need is a SIM card with a local number, and a pay-as-you-go plan. Even better, if I can order it online and have it shipped to where I’m staying in New York, so it’s ready for when I arrive.

I thought T-Mobile had this (saw it in a store in NYC last I was there), but I can’t find phone-less pay-as-you-go services on their site.

Does anybody know of such a service?