A former Arts Teacher of mine took a trip to Portland from the Seattle/Everett area and during that trip he purchased a Powermac Cube. I forget if that was the main purpose of the trip or not but he did avoid the sales tax. I've thought about it myself but the price of gas now a days and the pollution I would be releasing makes me think twice. I could always buy a bus ticket or something. Actually, with todays better fuel efficiency and pollution controls in cars it might still work. Who knows.
If I am not mistaken you are taxed by where your billing address location is but you can have it shipped to a different address. You want it shipped to an Apple Store? Well, that might present a problem. Or it might not. As already suggested call Apple on that one.