Depending on how long you are staying but I use AT&T's go phone (PAYG) sim.
Get the international rate option - buy the top up on line for a 5% discount and you get pretty good rates.
I have my number on friends and family for the international choice - its cheaper than calling my UK Vodafone number from a BT Landline!
I will probably move to TMobile USA to take adavantage of the data prices for my iphone.