At least seen here with a 8400m (Dell Symptoms pretty straightforward - boots fine to greeter then nothing but a Desktop image & cursor, both on unity & GS. Nothing is possible once leaving the greeter. Same on current live image > 'Try Me'
Works fine with Classic (No Effects) & nvidia-* drivers Seen with both rc5 & 6 https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1080389