April 2013, I had the same issue, Ubuntu 12.04 LTS, also after using Ubuntu Tweak.
Here's how I fixed it :
It's almost the same as Githlar's first post.
My video card is AMD ATI Radeon HD and I used wired internet.
(1) I chose to start the login console ;
(2) then tried to reinstall video drivers :
sudo apt-get update && sudo apt-get install fglrx and of course, returned "yes" to install the packages to install.
This installed 3 packages : dkms, fglrx and fglrx-amdccle. An error has occured while installing dkms but it hasn't interrupted the installation.
The problem's fixed, but still have that resolution issue with lightdm login screen (unsupported by the monitor).
Hope it may help people with similar issues.