From a post I made at Fark.com:
Zombies represent a fear of conformity, of being “eaten alive” by the shambling hordes of the collective. For Americans, this is one of our greatest national fears; losing our individuality, our “freedom” if you will.
Also, much as vampires represented an escape from the rigid Victorian norms of the 19th century, zombies represent a derivation from our generally boring and comfortable lives. Zombie stories require survivors to feed into the escapist fantasy they represent; you don’t really imagine being a zombie, you want to be in a world where the rules of society are gone, replaced with the simple, natural rules of survival.
Zombies are the creeping uniformity of the modern world, zombie worlds represent the ultimate break in the structure of society.
Basically, until the mass populace comes to fear something more than conformity and desire something more than a removal of the burdens of the social contract, zombies will always be in vogue.