Depending how long and what you want to use your phone for (incoming or outgoing calls) in the US, and want to save some money, do this.
1) Obviously make sure your phone is 1900Ghz compat.
2) Walk into a T-Mobile phone shop when you get there and buy a prepaid sim.(Usually about $65.00 with $50 of free calls)(2 refill you must buy refill cards, as T-Mobile wont allow Intl CC to refill via their automated service)
3) For incoming calls, (If your phone company will allow 2 diverts, Divert your Mob to home phone, then to US mobile number. If not, then put a divert on the home phone and tell people to ring that) You end up paying normal Mobile + home Intl rates, which some are as low as 3-5c per min)
4) For outgoing calls, just use the credit.
If money is no object, please disregard the previous statements.
PS. If you go to Mexico dont use the AUS mobile their. I found out the hard way Optus charge about $7 / M. A 5 minute call back to aus ended up being $35.00 !! Also, local calls in the US are not charged at a normal rate. Another pitfall....
Hope this helps
Rob