This particular bit of information can be found in their press release earlier this morning, where Apple made a statement about their Friday launch plans. The statement which showed off that particular clause was “iPhone is available by reservation only in China, Hong Kong, Japan and US stores in tax-free states.”
This would mean that the quartet of U.S. states affected which will feature Apple retail stores over there as well will include Oregon, Delaware, New Hampshire, and Alaska. Montana happens to be the fifth U.S. state that does not have a statewide sales tax, but we can rule that out since Apple has yet to set up a single retail store there. Chances are scalpers might want to take advantage of a walk-in sales, so this is a small way of minimizing that from happening. [Press Release]