Comment 43 for bug 247376

goto console, alt+f1 or Ctrl+alt+f1,
login,
cd /etc/X11

open your xorg.conf by vi or nano,

goto device section,
change driver from "fglrx" to "ati" or whatever you have...
you may also add disable "dri" to modules section

try startx,
if your xwindow starts, its ok.

restart your system.

btw, i must say that, ubuntu guys should avoid this, even in alpha-beta stages....