I just made a complete upgrade/dist-upgrade of my Linux Mint Debian. After rebooting, I was prompted to shell login tty1 without a GUI/Gnome on tty7 running.
sudo xinit showed some error messages like:
module abi major version doesn't match the server's version failed to load module vboxvideo
I added Virtual Box to the repository list:
# /etc/apt/sources.list deb http://download.virtualbox.org/virtualbox/debian squeeze contrib non-free
and reinstalled the guest additions
apt-get install virtualbox-guest-x11 virtualbox-guest-additions
after rebooting, now the GUI is back.