The xserver-xorg-core version 1:1.0.2-0ubuntu10.3 is broken.
I (tseliot) have sent an email to the chief of the team which maintains the Xserver.
The recent update to xserver-xorg breaks the Xserver.
Please wait with upgrading Ubuntu Dapper until xserver-xorg-core_1.0.2-0ubuntu10.4 hits the servers.
UPDATE: the problem has been fixed but you should wait to upgrade until xserver-xorg-core_1.0.2-0ubuntu10.4_i386.deb is uploaded to the ubuntu servers.
If you want to upgrade using the terminal then issue this command when the new package is uploaded to the ubuntu servers :
$sudo aptitude update && sudo aptitude upgrade
There are multiple (temporary) stickies (in different forum sections) linking to tseliot’s sticky. When an affected user is able to go to the forums (for example using a livecd) he/she will probably find tseliot’s sticky easily.
Here’s the bug report. The developers have fixed this really quickly.
A related blog post with a possible (future) solution to this kind of problems.
official information :