Can I buy a prepaid SIM card in the USA to use while travelling there?

I'm going to visit a few friends in the USA and generally travel up and down the East Coast.

I'd like to buy a prepaid SIM card for my GSM phone so I can call friends in the US, friends in the UK, and parents in Taiwan. Cheap international calls please! (Well, cheap by mobile standards, i.e. less than 20 cents/minute.)

Is it easy to buy a prepaid SIM card in the US? Are they sold at airports, shopping centres, convenience stores, etc?

Here in the UK, you can buy a prepaid SIM card almost everywhere and with the minimum of hassle (no documentation, no ID, just £10 please). Well... anyone who's watched Bourne Ultimatum would know how easy it is =p. Is it the same in the US?

Thanks! ^^
My phone is triband and unlocked, so it will definitely work as long in the US. All I need is a SIM card.
*definitely work in the US* (excuse the weird typo above)

Unlike in Europe, prepaid SIM cards do not exist in the United States. You'll probably get a funny look if you ask for one. We have SIM cards, but each phone comes with one and it is used as memory.

You'll probably have to use some type of calling card. I'll try to find more info for you and edit this answer.

EDIT:
http://www.cellularabroad.com/usppsc.php
http://ideas.4brad.com/best-company-each-country-buy-prepaid-sim

Why not just purchase a cheap prepaid phone, when you're done?

If your friends in the US have AT&T and you purchase an AT&T, your calls would be free. Some of the other carriers do the same thing, free calling between users on the same network.

Read the site below, apparently prepaid SIM cards are available.

As for prepaid calling cards, check out the second site.