In the upcoming patch Druids will be gaining a new spell, Barkskin. Caydiem from Blizzard explains what its purpose is.

I believe it's an answer to "what should I do as a healer in a group when I get aggro". Many players shift into bear form now, which is decent but prevents you from casting or using items entirely. This spell should allow the tank to regain the threat and keep you running and performing in your role. :)
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- Caydiem -

The complete thread discussing this spell can be found on the blizzard forums here.