Comment 8 for bug 971613

Related:
https://bbs.archlinux.org/viewtopic.php?id=140287
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=667499

The suggestion in those threads that this relates to a mangled setup of display drives seems likely, having installed the restricted drivers recently (gnome-shell is also unhappy at me, for what I suspect is the same reason)