Comment 14 for bug 57153

Recovery Instruction (with working internet connexion)
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Run your great GNU/Linux Ubuntu Dapper Box, wait ....
Get a terminal ! try Ctrl-Alt-F2, Ctrl-Alt-F3, etc ..

login !

type now :
sudo apt-get install wget

Type this on ONE line :
sudo wget -c
http://archive.ubuntu.com/ubuntu/pool/main/x/xorg-server/xserver-xorg-core_1.0.2-0ubuntu10_i386.deb

Now the recovery fix itself ( reinstall the previous package )
dpkg -i xserver-xorg-core_1.0.2-0ubuntu10_i386.deb

Then :
/etc/init.d/gdm start

The End
Have Fun
http://gistlabs.homelinux.net/

PS: You can download the file on another computer : go there :
http://packages.ubuntulinux.org/cgi-bin/download.pl?arch=i386&file=pool%2Fmain%2Fx%2Fxorg-server%2Fxserver-xorg-core_1.0.2-0ubuntu10_i386.deb&md5sum=d359edda430ad9ac6d119e5a3bb213ad&arch=i386&type=main