I too was bitten by the Ubuntu 11.04 upgrade in Parallels Desktop 6 for Mac.
*I booted normally and hit the "M" key to "manually" fix the problem occurring at boot. This got me to a bash shell.
*I chose to reinstall the Parallels Tools from the Parallels menu so the ISO file would be mounted to the virtual DVD device.
*Next, in Parallels, I mounted the image in Ubuntu with the command "mount -o exec /dev/cdrom /media/cdrom" (or whatever directory in /media or /mnt that you wish).
*After the image is mounted in Ubuntu, run the command "cd /dev/cdrom; sudo ./install" This will start the installer script and allow you to use the script to remove the Parallels Tools.
*The script should complete with no problems and ask you to reboot.
*Reboot your VM and select "classic" at the login screen since you will be unable to run Unity without the Parallels Tools video driver.
This allowed me to at least use my Ubuntu VM without rebuilding. I hope it saves someone the hassle of starting over.
Last edited: Sep 13, 2012