It also acts strangely with the XRandR extention (that allows dynamic screen geometry changes):
ajm@RS-031:~$ grep RandR /var/log/Xorg.0.log
(**) Option "RandR" "on"
(**) RandR enabled
ajm@RS-031:~$ xrandr
Xlib: extension "RANDR" missing on display ":0.0".
If you're intending to run X on a laptop, avoid Intel 915GM embedded graphics.