Black screen when password is entered

When I enter my password correctly, the screen turns black for several seconds before returning me to the login screen. Sometimes the mouse cursor will be drawn on the screen and respond to input when the screen is black, other times boot messages will be displayed.
If i change to a tty I can log in successfully.
/var/log/x-0-greeter.log contains a line that reads "[+0.58s] MESSAGE: Couldn't find primary device"

ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: unity-greeter 12.10.4-0ubuntu5
ProcVersionSignature: Ubuntu 3.5.0-17.28-generic 3.5.5
Uname: Linux 3.5.0-17-generic x86_64
ApportVersion: 2.6.1-0ubuntu3
Architecture: amd64
Date: Sun Oct 21 10:54:27 2012
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release amd64 (20121017.5)
ProcEnviron:
 TERM=linux
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: unity-greeter
UpgradeStatus: No upgrade log present (probably fresh install)