If you have an iPhone 5, or any other unlocked iPhone for that matter, and plan on traveling to the United States, you may want to save yourself the hefty fees that come with roaming, and get a local, pre-paid or Pay As You Go SIM-card for your service. There aren't too many options -- American carriers love tying everyone into 2 year contracts -- but there are some worth considering.
Official word from T-Mobile is that they will be supporting the iPhone 5 just as they have any other unlocked iPhone. You can use it on a post paid or prepaid plan with no restrictions.
T-Mobile USA plans on carrying nano SIM cards that will be compatible with the iPhone 5 in mid to late October. If you're using an iPhone 4 or iPhone 4S you can already pick up a micro SIM from T-Mobile. If waiting until mid to late October isn't an option for you, you can always buy a SIM cutter or cut your own down. It's hit or miss and we'd personally recommend waiting for the official one to hit.
While the iPhone 5 will work on T-Mobile USA, they could not confirm that the iPhone 5 would run on their LTE network. It most likely won't but you'll still be able to access their 2G network in most places, and their 3G/4G network in place they've begun supporting. (Most of T-Mobile 3G is on AWS spectrum, which is not supported by the iPhone.)
T-Mobile's prepaid plans start at $50/month and go up from there depending on how much data you need.
If you want to bring an iPhone 5 to AT&T you'll be out of luck when it comes to data if you want a no-commitment prepaid plan. While voice calls will work, AT&T has designed data not to be accessible. If you put a post-paid AT&T SIM into an unlocked iPhone 5 you will have access to data, which explains why CDMA iPhone 5's can use AT&T's network as long as the SIM you're inserting is from a contract account. AT&T uses IMEI information to detect account type and currently will not allow access to their data network on an iPhone 5 without a monthly plan.
If you're only in the United States for a short period of time and you're okay with only having voice service, it'll get you by but T-Mobile will almost always be a better option when it comes to using an iPhone on a Pay As You Go plan.
We're still polling the regional carriers and MVNO to see what other alternatives might be available. If you know of any, drop them in the comments, and we'll update as more and better information comes our way.
If you've had any luck activating an iPhone 5 on any other prepaid carrier in the United States, let us know in the comments!