Hi there, I'm having the same problem in FC6. The Red Hat Graphical boot seems to be working ok, as it completes fully. After completion, the screen flickers a few times and my monitor displays 'Signal Over Range'. This seems to imply a misconfiguration of x11, but this is odd as it should use the same as the non-xen kernel.
I have read somewhere that this can be caused by having the refresh rate set too high; apparently some older monitors can't support anything above 60Hz (although I seem to remember this one working at 75...)
I would try ssh'ing into the box to see if there is a working terminal, but this is the only linux box in the house and I can't seem to get windows alternatives working.
xorg.conf looks like this;
# Xorg configuration created by system-config-display
Identifier "single head configuration"
Screen 0 "Screen0" 0 0
InputDevice "Keyboard0" "CoreKeyboard"
Option "XkbModel" "pc105"
Option "XkbLayout" "gb"
Viewport 0 0
Modes "1024x768" "1024x768" "800x600" "800x600" "640x480" "640x480"
The only thing that seems to be of note is that all the screen resolutions are listed twice; im loathe to change them as I can't yet find any way of booting to the terminal, so could be screwed if I can't boot to gnome in the normal kernel.