Related to http://forums.linuxmint.com/viewtopic.php?f=189&t=101866, it seems this only happens when Nvidia twinview is enabled. I'm using this for a second screen. As long as twinview is enabled the machine can not be shut down properly. But it can when reverted to a single screen. This wasn't the case for UP3.
Does somebody else has this problem ? And if so, is there any way to fix it ?