In Ubuntu Oneiric, the default desktop session is the Unity Desktop (or Unity 2D depend on your hardware capabilities). If you have set the computer to auto-login to your desktop whenever you boot up, it will always login to the Unity desktop, even though you have installed Gnome-shell or classic Gnome. It doesn’t matter which desktop session (Gnome shell, classic gnome or Unity) you are using before you shut down, because on the next boot up, it will automatically login to the Unity desktop again. If you have Gnome shell installed, here’s how you can change the auto-login to Gnome shell instead of the Unity desktop.
Open a terminal and type:
sudo /usr/lib/lightdm/lightdm-set-defaults -s gnome-shell
That’s it. Simple fix. Big differences. Less annoyance.