Pre-Paid US SIM Card

We are going back to the US for 2 weeks and I would really like to buy a US pre-paid SIM card as I usually run up a pretty high bill on my UK phone when I am in the US and would like to avoid this from now on.
Has anyone ever bought one? I am looking on some websites but I am not sure I am finding what I am looking for. For example, I found a website (telestial.com) that does SIM cards that cost $29 but only have $8 worth of airtime on it. What is the the other $21 buy me?
The call rates seem pretty cheap, at 15 cents per minute for US domestic calls, for both incoming and outgoing, for the first 5 minutes and then 9 cents afterward.
I have a Nokia 6021 that is triband and I believe, unlocked, as it is on O2. Its my company phone so I am planning to take the UK SIM out and use this one in the US.
Any help would be appreciated!

man, telestial cards are very overpriced if you ask me...when I was young and naive and had never been to the UK before, I paid $40 for a Virgin Mobile PAYG SIM through their site that I could have picked up in the UK for £5 or less!

AT&T have 'Go Phone' SIM, for which there is a small charge to get started, and then IIRC $25 credit will last 3(?) months, and can be topped up online (credit is then preserved). I don't yet have one, but intend to get set up next time I'm in the US.

BoostMobile by Sprint & T-Mobile both use Sim cards for their pay as you go.  T-Mobile knock $5 off the price of their sim (preloaded with 10 minutes of airtime) when you buy online.  For Boost, it looks like you can only buy it with a phone or sim only in store (not sure, am going to call a Sprint shop tomorrow to find out more).

Boost charge $0.20/min to UK landlines & $0.30/min to UK mobiles.  T-Mobile charge $1.60/min to UK.     AT&T charge $0.39/min at least I'm assuming that they class the UK as part of Western Europe.

T-Mobile's the only one I've seen thusfar that explicitly mentions that they'll sell the SIM without a handset...

AT&T have 'Go Phone' SIM, for which there is a small charge to get started, and then IIRC $25 credit will last 3(?) months, and can be topped up online (credit is then preserved). I don't yet have one, but intend to get set up next time I'm in the US.

I can vouch for the GoPhone - although I didn't set mine up as a PAYG on a visit to the US, so can't really speak for the speed at which they set you up.  I was a Cingular customer from 2002-2006, and switched to GoPhone when I moved to the UK.  I've topped up with $100 twice now, which has lasted me through, let's see, 12 weeks worth of visits to the US since September 2006.  It's really basic - 10¢ per minute, 10¢ per text - and I've never had a problem with customer service or anything.

T-Mobile's the only one I've seen thusfar that explicitly mentions that they'll sell the SIM without a handset...

Yep..the T-mobile one seems like the best option, and I have confirmed that my phone is unlocked so it should work.
$1.60 a minute to the UK seems steep but I need it more for making calls within the US, so if I buy 100 minutes or whatever, that should get me through.
Too bad I cant order it on line to get the $5 savings..I doubt they will ship to here and there may not be enough time for it to arrive, and the shipping charge will defo wipe out the on line savings

Thanks everyone for your help 

Does anyone know if you need proof of a US address to buy a US SIM card like you do to buy one here in the UK?  (At least I seem to recall having to supply some kind of evidence, maybe I'm wrong) I only ever had a l-t contract in the US. 

I really want to get a SIM card for an upcoming trip to the US to be able to coordinate with friends & family (US #s) but also with DH (on his UK mobile - sigh, that $1.60/min isn't great...)  My UK phone is unlocked - I use both Talk Talk and Mobile World SIM cards here, but neither will work in the US, although the phone itself will.  I still have a US drivers license, but we sold our house so don't want to use that address.  Could use mom's address but have nothing in my name...

You need a proof of address to buy a PayGo sim card here? Is that new?