After updating Virtualbox to 4.2.12 and updating the guest additions on my Ubuntu 12.10 the guest suddenly started having problems with the graphics driver (X.org wouldn’t start correctly, only starts in “rescue” mode). I did a bit of googling, and it seems other users are also affected (mainly 12.04 and 12.10 Ubuntu), and it seems to affect people who upgraded from virtualbox guest addditions 4.2.10 -> 4.2.12
Here the quick and dirty workaround to the problem: download the 4.2.10 guest additions and use them.
mount -o loop VBoxGuestAdditions_4.2.10.iso /mnt
Will need to reboot or make sure the new modules are loaded and restart X afterwards.