I installed the Unity desktop and TightVNC using these directions. The window comes up in VNC, although because I'd started the VNC Server as root, I seem to be logged in as 'guest'. Please note that at the moment I only have console access as either guest or via SSH; I don't have physical access to the machine to login as root and make changes in the GUI. I've been starting the VNC server as root.
In the top right of the screen, it displays that I am logged in as Guest. By clicking on Guest, I can select another user to log in as, but it doesn't prompt me -- it simply turns the screen black for a second and kicks me back to the guest user.
I'm using gdm, gnome (using the unity 2d skin, I think, but I don't use GUIs much), and tightVNC. This server is hidden deep inside a private network and I'm not too worried about the security risks of connecting to VNC and getting a root shell.