I was having serious Xorg freezing problems with my ATI Graphics card - but Unity was loading.
So I removed it in favor of a new Nvidia card. During the initial boot-up before I could install the Nvidia drivers, I got a message that said "Your hardware can not support Unity, so you will be loaded into Classic Ubuntu".
Since then I've installed the official Nvidia drivers and everything is running great, but I can't get Unity to restart.
Every time I reboot or login / logout I still get Gnome, even when I'm picking "Ubuntu" from the login menu.