I did my research on this about 4 months back, so not sure it's totally current, but for what it's worth hear it is.
Only 3 and everything everywhere and a particular flavour of Vodafone ( I'll get the name and post back later ) allow tethering out of the box on an all you can eat basis. All the others will allow tethering, but you have to pay an extra charge for the data. Cost depends on how much used. We took the view that we would prefer the one up front payment per month and not worry.
I know that up to 8 weeks ago you couldn't tether to a 3 pay as you go sim, as son number 2 visiting from Australia couldn't find a cost effective way of doing it.
As an alternative you might consider an unlocked Mifi unit available on eBay. You can buy prepaid data sims online, and this alternative has the advantage of allowing up to 5 units to tether to it. The iPhone tethering only allows 1. IMHO it also has a better 3G antenna so we have had a signal on the Mifi when the phones didn't have one. Also as its opened you could carry a selection of prepaid data sims with you so increasing your ability to connect. You can also buy foreign based data only sims which enable browsing in foreign countries paying at local data rates. Use the apps Viber or what's app, and you have über cheap phone calls from most places. Our setup has worked all over Europe and I'm pretty sure it will work in the states. The Maldives also presented no difficulties. We did have problems in Australia as they use a different frequency for the 3G, so we bought an Australian Mifi and off we went. Good luck
Talkmobile is the name of the other network I mentioned. I'm pretty sure it's piggy backed on Vodafone.
Only 3 and everything everywhere and a particular flavour of Vodafone ( I'll get the name and post back later ) allow tethering out of the box on an all you can eat basis. All the others will allow tethering, but you have to pay an extra charge for the data. Cost depends on how much used. We took the view that we would prefer the one up front payment per month and not worry.
I know that up to 8 weeks ago you couldn't tether to a 3 pay as you go sim, as son number 2 visiting from Australia couldn't find a cost effective way of doing it.
As an alternative you might consider an unlocked Mifi unit available on eBay. You can buy prepaid data sims online, and this alternative has the advantage of allowing up to 5 units to tether to it. The iPhone tethering only allows 1. IMHO it also has a better 3G antenna so we have had a signal on the Mifi when the phones didn't have one. Also as its opened you could carry a selection of prepaid data sims with you so increasing your ability to connect. You can also buy foreign based data only sims which enable browsing in foreign countries paying at local data rates. Use the apps Viber or what's app, and you have über cheap phone calls from most places. Our setup has worked all over Europe and I'm pretty sure it will work in the states. The Maldives also presented no difficulties. We did have problems in Australia as they use a different frequency for the 3G, so we bought an Australian Mifi and off we went. Good luck
Talkmobile is the name of the other network I mentioned. I'm pretty sure it's piggy backed on Vodafone.

