I live in the UK and will be taking my iPad to the states so I will only have a UK sim and roaming charges are crazy.
I intend to just buy a local sim there to save me money so I hope I don't have any problems walking into one of the phone stores and picking one up.....
You don't have the right bands for Verizon LTE. AT&T 3G may be OK, depending on payment method (there's been issues with them and foreign credit cards).