By any chance did you buy the 17 inch vice 20 or 24 inch version? Of course there are occasional lemons about with any purchase of Mac equipment. But my Mac colleagues who have the (24 inch) model seemed to have experienced no problem whatsoever. Yet my sister-in-law with the 17 inch model has had several.

Good luck in your dealings with the Store or Customer Service. Every once in awhile, you actually do get an upgraded replacement.