It depends on which US state you bought the iPhone from in order to get the $649 USD as the final price out the door for the 16GB unlocked version.
In Texas (where I'm located), the sales tax here is 8.25% & with the unlocked iPhones only available directly from Apple, the final cost for me if I'm buying a 16GB unlocked iPhone 4S will be $702.54 USD ($649 + $53.54). Therefore, in my case, the price isn't any much different if I buy the iPhone 4S here in the US or in Australia. 😉