FYI, if you're not using xdm/kdm/gdm but are instead starting the X11 server manually with startx (which is what I usually do) you might have experienced brokenness in Debian unstable recently:

Fatal server error:
Unrecognized option: /etc/X11/xinit/xserverrc

This is already reported as bug #482425 and #482527 and should hopefully be fixed soon, but in the meantime this patch against /usr/bin/startx should work around the issue:

--- /usr/bin/startx.orig 2008-05-26 18:21:26.000000000 +0200
+++ /usr/bin/startx 2008-05-26 18:21:36.000000000 +0200
@@ -107,9 +107,7 @@
if [ x"$server" = x ]; then
# if no server arguments or display either, use rc file instead
if [ x"$serverargs" = x -a x"$display" = x ]; then
- server=$defaultserver
serverargs=$defaultserverargs
- display=$defaultdisplay
else
server=$defaultserver
fi

Hope that saves some people out there lengthy investigations and hassle.