Recently I had an Xvfb session in place, using x11vnc as a connection method. Upon login, I saw the following errors:

Error activating XKB configuration.
Probably internal X server problem.

X server version data:
The X.Org Foundation
60802000

If you report this situation as a bug, please include:
- The result of xprop -root | grep XKB
- The result of gconftool-2 -R /desktop/gnome/peripherals/keyboard/xkb

Ultimately the problem was fixed by adding the -kb option to the Xvnc line in the /etc/X11/gdm/gdm.conf file:

...
[server-Standard1]
name=Standard server
command=/usr/X11R6/bin/Xvfb -kb -ac -screen 0 1280x1024x24 -pixdepths 24
flexible=true