I visit the US regularly from the UK and the best deal at the moment seems to be...

www.net10.com

No daily fee, all calls 10c/min and you can currently get a $29.99 starter handset which is bundled with $30 airtime so that would be all you need!

However...

I use T-Mobile To-Go Prepaid service. You can also pick up a handset in Best Buy, Target etc for $30 but will need to add some airtime. $25 gives you 120 mins and $50 gives you $400 mins. This works better for me as the T-Mobile service lets you text back to the UK whereas the Net10 one doesn't. Texts are currently 10c but I think these are due to double shortly to 20c

Don't use either to make international calls. T-Mob, example, is $1.50/min!! I use a CVS (Pharmacy) Sprint calling card. I have used a few in my time but the Sprint card is the only one that charges the same 6c/min for calls to UK Mobiles as well as landlines. Others charge extra for Mobile calls.

So I can make a call from my US mobile to a UK mobile for the mobile networks 10c and add on the calling card 6c. Works great for me.

Now if only the US had a decent data plan for Pay as you Go. In the UK you can pay £10 and get 1GB data on the "3" network. Something like that in the US would be superb!

Rich