Note that if you DONT declare it Customs can spot a US bought laptop so you may have to pay extra to get it through customs.
A fourth to add to neildownx's list, which is support. If it goes wrong during its warrenty you may not be able to get support over here (for example a replacement if it really goes wrong).
For the little that you may save I dont think it is worth it.
You should also be aware that any product from the USA will tend to eat enormous meals and will also speak with a mouthful of food, thus making it very difficult to understand (and possibly very messy).