Unlike the UK, US cell phones get charged for incoing calls (it won't be any more than a domestic incoming call, but the minutes may add up).

Also, US prepaid minutes expire after 30-90 days (Unlike UK prepaid services).

You may want to look into T Mobile's service (www.tmobile.com); they use GSM, so if your phone is unlocked, you may be able to just buy the SIM. ebay has a bunch of sellers wit just the SIM.

There are lots of storefronts that sell cell phones in NYC; if you walk down Broadway, you should come across a bunch.

Virgin also has a prepaid service (It uses Sprint's system); starter kit (Phone plus some minutes) should be about $70. Go to www.virginmobileusa.com for more information; the site also has a store locator to direct you to a retailer that sells the phones. Calls are 0.25/minute.