After installing and rebooting, my X server wouldn’t start. I dropped into a virtual terminal only to see that Xorg could not find any screens to use (Fedora 12 has no xorg.conf, it use HAL to detect the displays instead). This happens in both 32 and 64 bit versions.

It seems this is a general problem that can be fixed by creating your own xorg.conf with the required settings. The second link I provided has the fix.

I haven’t tried it myself yet but hopefully a new version of Virtualbox will soon fix the problem. It did occur to me that it would be nice if there were an easy way to uninstall guest additions to fix problem like this, at least then I could make a temporary fix to investigate the problem.