I use XDarwin in fullscren mode and have configured it to switch back to Quartz
if I hit command-option-A (the saem sequence as is used by Apple's X11).

It usually works fine, but every once in a while, the escape sequence doesn't
work any more.  When this happens, command-option-A behaves just like Alt-A (I
can never remember which key is command and which is option, but one of them is
actually Alt and the other is a funny symbol: the one with the funny symbol
seems to be ignored).

I have not noticed any particular pattern yet of when this happens.  Sometimes
after a few X11 manipulations (typically switching workspace within my
window-manager), the command-option-A works again, other times it gets stuck
this way.  Since command-option-A is the only way I know to get back to Quartz,
I tend find myself having to exit from the X session.

It would be good to have a little executable that could force the Xserver to
switch back to Qaurtz.  It could be used similarly to the XDarwin icon on the
Quartz side.