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: