Narelle,
We had a month in the States earlier this year and went to a phone shop and brought a Simple Mobile sim card. They set up my iPhone for me so it was working straight away. Cost was $105 but we had unlimited data, 40 hours international calls and unlimited national calls.
We had no problems with coverage and used it On the mainland and Hawaii. It was great having the freedom to call home when ever we wanted plus the data was Bonus for checking flights, tours, emails, banking tc....
Heading over in May 2013 for another month and plan on doing this again