I've done this a couple of times now, I bought a Powerbook a few years ago and recently back in May or March I bought my 17" MBP over in the US. If you're going over there especially to buy a new MBP or macbook I'd say it's not really worth the bother as you'll spend what you'll save on the flights. However if you make a holiday of it then I reckon it is worth it! Just make sure you either take a laptop bag either with you or buy one and carry it back in that bag and mail the receipts and if you want the packaging back to the UK (the packaging might help resale value if you decide to punt it on Ebay at a later date)
I personally don't think it's even worth thinking about getting one shipped from the US to the UK - You
WILL get hammered by the courier company for VAT + their admin fee. Every time I've had stuff sent over from the US I've been stung by Fedex, UPS, DHL - I think they make a point in declaring goods to the Customs as it's a nice and easy money earner for them with respect to their admin charges (Fedex charged me an extra £13 as their fee!)
I sent my MBP box back home and it cost $15 with USPS and it only took about 3 days to arrive!
I think, I might be wrong, but NY has a fairly high Sales Tax? If you can, fly to Philidelphia, hire a car and pop about 30-40 mins down the road across into Delaware where there isn't any Sales Tax and there is an Apple Store in the mall just over the state border - That's what I'll be planning to do in the near future!
If you do get hammered for VAT on your return there isn't any import duty on computer equipment so it'll be a straight 17.5%, but having said that each time I"ve returned from the US I've hardly even seen any Customs officers on duty so you'd have to be very unlucky to get stopped, but as mentioned above if your morals think it's wrong then don't do it - personally, I think I pay enough tax as it is so I wouldn't think twice about doing it!
ps Don't forget to get a mains plug adaptor for when you get home! and don't forget the US keyboard layout is slightly different to ours but you get used to it quite quickly and the Apple warranty on laptops is worldwide!
