lightdm changed it's syntax with the most recent version and also no longer requires a configuration file causing the automatic login normally configured for the 'ubuntu' user to stop working.
Derivatives will still use /etc/lightdm/lightdm.conf but you can't rely on the contents anymore and just replace a string with a regular expression. Standard ubuntu doesn't have /etc/lightdm/lightdm.conf.
This failure is with the mythbuntu lightdm conf but will happen on all cases of lightdm right now.
DistroRelease: Ubuntu 11.10
Package: casper 1.272
ProcVersionSignature: Ubuntu 3.0.0-7.9-generic 3.0.0
Uname: Linux 3.0.0-7-generic i686
Date: Tue Aug 2 04:38:11 2011
LiveMediaBuild: Mythbuntu 11.10 "Oneiric Ocelot" - Alpha i386 (20110802)
UpgradeStatus: No upgrade log present (probably fresh install)