Sonja, remember that there's a difference between Defense SKILL and Defense RATING. When people say you need 540 Defense, they mean the skill. 540 Defense equals ~689 Defense rating.
It's also worth noting that you only need 540 Defense for raid instances -- Naxxramas 10 and upwards. If you only do 5 man instances, the most you will need is 535 Defense (~666 Defense rating).