Comment 79 for bug 882817

I tried Alex's fix (disabe session management in compiz) and it may have made the hangs less frequent, it has not solved the issue on 3 different sets of hardware.

As an aside, some dual-boot testing on a couple of machines with Mint 12, using Cinnamon and Gnome Shell sessions results in zero shutdown hangs. AFAICT the issue lies in something exclusive to "genuine" Ubuntu/Unity and the way it's doing a shutdown.