After using Ubuntu 12.10 for only about a week, and following only one update i could not manage to log into the Unity desktop. I managed to get around this with an obscure sollution found on the ubuntuforums.org (http://ubuntuforums.org/showthread.php?t=2049203), where a poster commented he had fixed the same issue by only installing gdm. This even worked for him after removing it later.
ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: lightdm 1.4.0-0ubuntu2
ProcVersionSignature: Ubuntu 3.5.0-18.29-generic 3.5.7
Uname: Linux 3.5.0-18-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.6.1-0ubuntu6
Architecture: amd64
Date: Tue Nov 13 22:40:09 2012
InstallationDate: Installed on 2012-11-12 (1 days ago)
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release amd64 (20121017.5)
MarkForUpload: True
ProcEnviron:
PATH=(custom, no user)
XDG_RUNTIME_DIR=<set>
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)
Closing assumed fixed.