I think it depends on which state you are talking about but RAC* advise that you should get an IDP anyway.
International Licenses are just translation documents. In Tasmania they only cost $AU30 or so. All they do is allow a police officer to identify that it is a valid drivers license on the spot instead of checking with some authority over the phone or back at the police station.
Like I said.. pay the 30 bucks and you reduce your potential stress a lot.
http://www.racv.com.au/wps/wcm/connect/Internet/Primary/travel/driving+overseas/driving+in+the+united+states about 8 years ago
In June/July/August last year I hired a car from Thrifty in New York. I did not require an 'Inetrnational License' to hire or drive the car. I drove from New York to Michigan, the nacross to Kansas,thenbackdown to Florida and up the coast to New York.
Twice I got pulled over by State Troopers for driving a 'little' over the speed limit (you know, its confusing somethimes between kms and miles/ hour) and was never questioned about needing an 'Internastional License' - though they did laugh a little at my strange accent, the fact i was an Aussie in the middle of "nowhere USA" and that my NSW Drivers License looked like it came from a cereal box :) about 8 years ago