This posting is to save time for anyone else who encounters problems in upgrading a guest from F17 to F18 using fedup. The symptoms are a system that boots in single user mode, but won't launch a GUI. In Xorg.0.conf, you will see "module ABI major version (12) doesn't match the server's version (13)". The cause is that it's not enough to do /etc/init.d/vboxadd setup; you need to mount the virtualbox guest additions disk again, and run the guest additions installation from there.
Best Wishes
Bob