raw egg white contains a substance called avidin that will bind to biotin (an essential B vitamin) and make it unavailable to the body. luckily, the yolk of the egg is high enough in biotin to make up for this. so if the egg is fed as a whole, then it should be fine, but feeing only the white can cause problems. this only applies to raw egg as far as i know, i believe that when eggs are cooked the avidin is deactivated.
i feed my kitties eggs on occasion and they love them. and eggs are the most digestable protien! when i give my cats (and my dogs) eggs i include the shell, dried and ground up fine.