Fixed on dapper-updates release of xorg-server_1.0.2-0ubuntu10.3.
Thanks for the hard work, guys!
(just for reference, this bug is not present in the current version of
xorg-server, present on Ubuntu Edgy)
** Changed in: xorg-server (Ubuntu)
Status: Needs Info => Fix Released
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XOrg dualhead regression
https://launchpad.net/bugs/36461