In all previous releases Revit was a bit too accommodating because it would let users put multiple ceilings within the same boundary without complaining about the redundancy. Obviously some parts of a building use multiple ceilings so they need to be able to exist in the same room or space. However the Auto Ceiling function also made it too easy to put multiple ceilings in the same room or space because it didn't complain or mention the redundancy. I complained about this in an earlier post.
A quiet change in Revit 2011 exists now (documented thanks to Inside the Factory), a warning will occur if we use the Auto Ceiling function within the same boundary room or space.
It's nice to see issues like these get resolution even if they forget to take credit for them in the "What's New in XX" documentation. Little stuff can add up to annoying.