You can't buy one in the US without a permanent address and social security number (i think), also your locked into a contract which you would have to cancel, also you cannot unlock any iphone that ships with 3.1. Also iphone warranties are not international so any problems you have with the phone will not be resolvable.
Your better off just biting the bullet and going to Apple.com/au and buying a fully unlocked phone for 1050$ for the 32gb, probably be cheaper in the long run.