Ever being upset that your Virtualbox Guest Linux OS is not able to display something else than a 4:3 ratio display?
This can happen after a dist-upgrade...
Start the guest and launch a root terminal.
aptitude update
aptitude dist-upgrade (this way, you are up to date)
/etc/init.d/vboxadd setup
That should do, if not make sure you have the correct kernel headers version installed.
Hint: After aptitude dist-upgrade... please reboot and only then launch vboxadd setup.
