Comment 60 for bug 1029790

Since Ubuntu team is just able to tell us to report that it doesn't work at each single version of the kernel, waiting for upstream to fix the trouble, I put here a workaround (tested on raring) to all those people whose got this 100% reproducible trouble since quantal release.

Add 'nomodeset' in GRUB2's kernel command line (using GRUB_CMDLINE_LINUX in /etc/default/grub) and run 'sudo update-grub2'
Put lightdm_delay init script in /etc/init.d/lightdm_delay
sudo chmod 755 /etc/init.d/lightdm_delay
sudo update-rc.d lightdm_delay defaults
sudo reboot

Basically this script just reload radeon module and start lightdm daemon 1 second after.