LCD panels are split into grades, based on how many stuck/dead pixels are on the panel from the factory. Grade A panels are guaranteed to have no dead pixels, and only a SINGLE bright pixel at the most - these panels are basically 100% perfect.
Needless to say, Apple don't use Grade A panels in their screens as they are significantly more expensive than Grade B - in the region of hundreds more. Now, being Grade B, you still stand a good chance of getting a perfect screen, but there is the possibility that you may get some pixel problems. Hardly anybody uses Grade A panels, which is why a lot of companies refuse to switch screens as they don't have a Grade A policy - in essence, 3 or so is within tolerance.
Apple are in fact, very good if people do ask for a replacement - they usually get it, especially on a new machine. However, you don't have a right to demand a pixel-perfect screen from Apple, they only swap them out of goodwill. Apple aren't making you pay for a Grade A panel - if they were, Mac prices would be even higher.
Personally, I've never had a dead pixel on my Apple hardware, and that includes 6 Cinema Displays over time, 4 laptops and iPod/iPhones.