Got this problem after mounting a CIFS/Samba drive through my wireless network. This appears to be a rather normal problem for lots of users. It was also hard locating a fix for my problem, as many users are using network manager as well.
I found a great fix though, which also is reported to work for network manager.
It's rather uncomplex and logical. The fix is moving the umount procedure to a earlier runlevel in the rc.d folders. The problem appears to be that the system shuts down the wifi before umounting.
Fix:
Go into /etc/rc.0 and /etc/rc.6, and move
user@system:/etc/rc0.d$ mv S31umountnfs.sh S14umountnfs.sh
Hope this helps :-)
More info is available here: http://blog.avirtualhome.com/ubuntu-shutdown-problem-cifs-related/
