There is no legal way to avoid paying sales tax. You just aren't being forced to claim it at the time of purchase.
No longer worth it for some to purchase from apple retail store.
Information on AppleInsider.com suggests you will actually be better off purchasing your MAC from a 3rd party seller than the apple store, even with the new promotion.
I was awaiting the promotion because with the student discount and the BTS promotion I was looking at about a $300.00 discount to the original price of my iMac (27" 2.7 @ $1700). With the student discount you are down to a $1600 price tag, with the free iPod which I would have sold to a friend for $200 ($30 dollars cheaper than retail price), I was looking at paying a total of $1400 for a 27" 2.7 sandy bridge iMac which is the cheapest you can possibly get the price down to no matter where you shop.
Problem is apple retail also adds tax, in Ohio I have a 6.5% sales tax which would add about 103 dollars to my total. Without the Ipod being offered I am now looking at a total price of about $1700. (minus student discount but adding tax). With tax before I would have been at $1500.
3rd party sellers offer this same iMac free shipping and printer rebate that matches apples for $1579. There will be no tax in my situation, so by ordering online I will actually save about $120. PLUS im guessing some of the 3rd party resellers will offer some sort of deal/match for the $100.00 itunes gift card apple is giving (if you handle the sale correctly).
Shopping at apple will actually be more expensive so if you are in the market make sure you check out pricing guides at www.appleinsider.com and consider sales tax into your purchase IF you are choosing to purchase at the physical store and maybe you too will not purchase from apple and save some money by going through macmall or a seller like it! Hope this helps some people!