After installing the nvidia drivers from the "restricted extras" menu (after adding the correct PPAs) you are given the option to install the Nvidia Proprietary Drivers.
These drivers install successfully, but upon restart, cause LightDM to crash consistently. You are not able to get to the login screen, as LightDM crashes before this happens. However, you can drop to shell and do things like purge the Nvidia drivers from the system.
This also happens to newly installed systems because (as of beta1) the proprietary driver is installed along with the OS but is not configured properly (bug 1079812)
DistroRelease: elementary OS 0.2
Package: elementary-desktop (not installed)
ProcVersionSignature: Ubuntu 3.2.0-32.51-generic 3.2.30
Uname: Linux 3.2.0-32-generic x86_64
Date: Wed Nov 14 20:36:50 2012
InstallationMedia: elementary OS 0.2 "Luna" - Beta 1 amd64 (20121114)
PATH=(custom, no user)
UpgradeStatus: No upgrade log present (probably fresh install)