Just a quick little bug I wanted to share with others here who have a cataclysm beta.
Basically, Polymorph: Rabbit won't break when the target takes damage. I haven't tested this extensively, but so far it's worked on all mobs I've come across. Regular polymorph is not bugged, and I don't know if any other polymorph types are broken.
Just make sure you deal loads of damage to the target, they heal back to full very quick!
Edit: Tested, it does not work on other players.