Some excerpts from the xorg log:

(**) intel(0): Option "PreferredMode" "768x576"
...
(WW) intel(0): Option "PreferredMode" is not used

Actually the option was used since without it the default mode is used.


(II) intel(0): RandR 1.2 enabled, ignore the following RandR disabled message
...
(--) RandR disabled