They do not because there are so many different tax rates - even within states and zip codes you may have different tax rates depending on the location.Thanks for the useful advice!
If going 30mins out of NYC makes it cheaper, I'm sure I could manage that. I totally forgot that tax in the US varies per state. Does the Apple online store include tax in it's prices?
If you go through Delaware - Christiana Mall (just off the interstate) has an Apple store and no sales tax, as well as a Best Buy that will sell at edu price if the Apple store doesn't. It's about 2.5 hours from NYC, however so the cost of going there just for a MBP may not be worth it; unless you plan to buy other things as well.
Your best bet is to buy it mail order and ship to a friend you'll be visiting. If you have a UK credit card, call them or stop by your bank and explain you want to buy something in the US and ship it to a US address you'll be visiting. Many US cards allow you to add a second address as a verified address, don't know about UK cards. I can't imagine it's that rare given the amount of business travelers that go back and forth regularly.
Amazon even takes Paypal which may be another option. Amazon is pretty good about that - I've had friends in Europe buy things with a Eurocard and ship them to my house.
Bring a UK uni ID and the Apple store probably would do accept it if they need an ID. My experience has been they don't check and I've often seen people buying it as a gift for a student get the discount.