In my previous comment, the test was performed with Ubuntu 8.04.1.
Now, testing as proposed by Bryce on 2008-12-20 with the live CD option
2. Download and test the Jaunty alpha-2 (or newer) Live CD,
(which includes a beta of the new xserver 1.6 as well).
Reported problem seems to be solved a
This problem is also reproducible with same
xrandr --output VGA --mode 1024x768
command when launched with a custom button (see configuration in attachment)
Hope this could help.
** Attachment added: "Custom button configuration which reproducise this
problem"
http://launchpadlibrarian.net/20