Since I posted to this thread in March 2013 I've been using a pay as you go sim from H2o Wireless (which is part of AT&T) on my trips to the US, am very happy with the price and service, and recommend it to everyone I know that visits the US.
Here's a link:
https://www.h2owirelessnow.com/mainControl.php?page=planMin where you can take a look around and see if this might be what you are looking for.
Yes it gives you a US number.
Yes, you have to top up a minimum of $10 every 90 days to keep the number active but you don't need to use the phone. I top up either when I'm in the US and using the phone, or from the UK using Skype to their 1-800 number (but you do need to use a c/card with a US address). $40 isn't a lot to keep the same number, and I do use a lot of that credit when I visit.
No, it doesn't work overseas so is purely for US.
No, you don't have to pay $9.99 for the sim on the H2o website which doesn't include airtime credit. I have ordered sims for friends and paid as little as 0.1c + postage on Ebay.com/Amazon.com.
I also keep an old phone to use in the US as it saves me from the (slight) hassle of having to get the US data settings every time I visit and resetting my phone on return to the UK. I like to have access to data as soon as I arrive, but if you were swapping out the sim its easy enough to use hotel/Starbucks etc wifi to have the settings sent to your phone.
I hope this helps, I do have a nano sim and a regular sim at home if anyone wants me to post them out. You can activate the sim from the UK if you have access to a US c/card.