I can verify this bug in Hardy Heron Kubuntu 8.04. It is caused by the network shutdown script running before the cifs mount is unmounted. The system hangs for a while waiting for cifs unmount timeout. This has been reported previously back in 2007 and onwards. I found a work-around by:
sudo mv /etc/rc6.d/S31umountnfs.sh /etc/rc6.d/S14umountnfs.sh
Note that this was working for me in Gutsy and Feisty.