When trying to boot the guest, I got the normal splash screen, but instead of the login window I just got a black screen with a blinking cursor in the corner. When I logged in on another virtual terminal, I found the following messages in the X log file:
This indicated that the driver was unhappy with the desired resolution for an increadibly helpful "unknown reason". Apparently, attempting to set the resolution used in seamless mode was failing, possibly because it was too high for windowed mode.
[...] (II) VBoxVideo(0): Not using built-in mode "1920x1170" (unknown reason) (II) VBoxVideo(0): No remaining probed modes for output VBOX1 (II) VBoxVideo(0): Output VBOX1 connected (WW) VBoxVideo(0): Unable to find initial modes (EE) VBoxVideo(0): Output VBOX1 enabled but has no modes (EE) VBoxVideo(0): Initial CRTC configuration failed! (II) UnloadModule: "vboxvideo" [...]
As it turns out, VirtualBox stores the last resolution in the machine description file in the guest property
$ VBoxManage guestproperty get Fedora /VirtualBox/GuestAdd/Vbgl/Video/SavedMode VirtualBox Command Line Management Interface Version 3.0.8 (C) 2005-2009 Sun Microsystems, Inc. All rights reserved. Value: 1920x1170x32
After spotting this, the fix was reasonably straight-forward:
fixed the problem.
$ VBoxManage guestproperty set Fedora /VirtualBox/GuestAdd/Vbgl/Video/SavedMode 800x600x32