halt problem in Ubuntu 11.04 amd64 and Dell Optiplex 990

Just add the option "reboot=pci" to GRUB_CMDLINE_LINUX in /etc/default/grub

http://ubuntuforums.org/showthread.php?t=1741668&page=7#62
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/768039