When Cataclysm launches sometime next year, there will be a big change to the stat system for all players. Maybe none bigger than to tanks though as Defense will be dropped and instead given through your talent choices. No more juggling to hit those last few points, or maximize health while lower defense back down to the soft cap. Ghostcrawler explained it in the forums:

Re: No hit juggling if no Defense Juggling
The reason we want to do away with defense isn't because it is a "cappable" stat. Hit is interesting in that you need a certain amount but no more and have to make gear choices accordingly rather than just gemming and enchanting your best stat everywhere and using the highest item level piece that drops without other considerations.

There are several reasons to get rid of defense:

1) It's a confusing stat with its "cap" that isn't really a cap.
2) It contributes to avoidance inflation.
3) It's not of the same value for all 4 tank classes.
4) If you have a bunch of dps that can't hit, you'll probably just progress more slowly. If you have a tank that can be crit there's a chance you can't progress at all.
5) It contributes to tank scarcity. Ideally an Arms warrior should be able to go Defensive Stance and grab a shield to tank a 5-player dungeon (though not a raid). When every dps warrior (though the same is true of all tank classes) needs a full set of tank gear to tank even easy heroic content, it's harder to fill your Dungeon Finder groups.

Ghostcrawler
Lead Systems Designer