What's an MMO without theory crafters?
Some industrious folk have been looking over tanking stats in PvP and found them to be nigh on useless.
Armor doesn't protect you from internal or elemental damage, shields and your defense rating only affect weapon damage.
Which means almost all the damage caused by any player in PvP is in noway reduced by your fabulous tanking stats. You are no more durable than any other player and in fact you're worse off because of all those watsed stats on your champion gear.
White damage is reduced.
For the full indepth read: http://www.swtor.com/community/showthread.php?t=155034
You can even happily test this yourself in a duel. Duel any other player, get them initially to sue their basic 0 resource attack on you. You should notice shield absorbs and damage reduction from armour will work as expected.
Now have them use any specials, you will notice very few if any absorbs depending on the class.