x11 Remote Desktop with Ubuntu 12.04
I got it working with:
gnome-session --session=ubuntu-2d
This will automatically start unity-2d-panel and unity-2d-shell.
In order to make this a slightly more forward compatible, I'll add that you can find the sessions in /usr/share/gnome-session/sessions/. You'll see files like ubuntu-2d.session, gnome-classic.session, etc. You'll then use gnome-session --session=ubuntu-2d. In other words, you'll just drop the ".session" from the name.