Visual artefacts after greeter

When the session starts to load after logging in, the screen gets corrupted (see attached screenshot). I'm using ATI hardware (RV620, Mobility Radeon HD 3470).

Steps to reproduce:
1. Restart computer (or lightdm)
2. Enter you password, press enter and observe

What happens:
Screen
(1) gets corrupted (screenshot)
(2) shows old state (for example a still from few moments ago when I was typing my password)
(3) goes black (not really corruption but not something that should happen)

Everything goes fine very rarely.

What should happen
The greeter UI disappears and perfect picture of wallpaper is drawn without any flicker or artifacts.

This started after upgrading unity-greeter to version 12.10.2-0ubuntu1. When I downgraded unity-greeter to 12.10.0-0ubuntu2 and tested this, corruption didn't happen.

ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: unity-greeter 12.10.2-0ubuntu2
ProcVersionSignature: Ubuntu 3.5.0-11.11-generic 3.5.2
Uname: Linux 3.5.0-11-generic x86_64
ApportVersion: 2.5.1-0ubuntu4
Architecture: amd64
Date: Mon Sep 3 21:34:53 2012
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Alpha amd64 (20120807.1)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=fi_FI.UTF-8
 SHELL=/bin/bash
SourcePackage: unity-greeter
UpgradeStatus: No upgrade log present (probably fresh install)