There is a long delay on logout/shutdown. It seems to be related to Unity-2d in Ubuntu 12.04. The problem's been reported in the past (#812104, #1013133, which were fixed), but, unfortunately, it still exists.
ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: unity-2d 5.12.0-0ubuntu1.2
ProcVersionSignature: Ubuntu 3.2.0-35.55-generic 3.2.34
Uname: Linux 3.2.0-35-generic i686
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu15.1
Architecture: i386
Date: Sun Jan 6 02:55:20 2013
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release i386 (20120817.3)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
LANGUAGE=en_US:en
TERM=xterm
PATH=(custom, no user)
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: unity-2d
UpgradeStatus: No upgrade log present (probably fresh install)
Status changed to 'Confirmed' because the bug affects multiple users.