Nothing happens. It stays the same orientation. Oh, now I'm maybe realizing that your post was perhaps a joke.
Haj: Some have reported that when they restart the ipad, it was fixed and can now go into landscpe mode. That didn't work for mine. I wish I would've known about this since I never would have upgraded. I hate using the ipad in portrait mode, it is especially annoyiny with the smart cover on it, and use the remote app daily.
The ipad3 also seems slower, opening and closing windows now. Perhaps it is the really awful zoom effect-hate that too. Also newstand seems even more annoying since I can't close the nytimes app with one click of the home button anymore.