Go post on the tanking forum that defense has a cap of 540 and imply that beyond that additional defense is wasted. I'd grab on to some kind of immovable object first. 
Ghostcrawler when someone said:
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The current cap is 540 defense skill. That's it, not very confusing if you know how to read and know how to add. Most of those skills are developed in grades 1-4.
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http://forums.worldofwarcraft.com/th...&pageNo=12#226
Edit:
With my warrior tank I try to have more than 540 because I like to be an easy to heal tank.
With my pala I collect everything else but not def because the selfheal helps alot to keep me alive
and gives the healer an easy job.
But raiding as a tank means to have more than one equip and decide from boss to boss.
You should have at least one for stamina and for avoidance.