Comment 1 for bug 773038

Installing the guest additions directly from the package provided by Oracle works:

wget http://download.virtualbox.org/virtualbox/4.0.6/virtualbox-4.0_4.0.6-71344~Ubuntu~natty_amd64.deb
dpkg -x virtualbox-4.0_4.0.6-71344~Ubuntu~natty_amd64.deb your_directory_name
mkdir iso_image
sudo mount -l loop your_directory_name/usr/share/virtualbox/VBoxGuestAdditions.iso iso_image
cd iso_image
sudo ./VBoxLinuxAdditions.run

It seems that there is an update from 4.0.4 to 4.0.6 which has not been shipped for the Windows version of VirtualBox.

Maybe this update could be included in the backports (?).