Yet it is so. Try it out. Install Ubuntu 11.10. Boot and login. You have GNOME 3 with Unity, no GNOME Shell.
Now apt-get install gnome-shell. Log out. In the login screen, choose "GNOME" as your session. Login. Witness GNOME 3 with GNOME Shell, no Unity. For full effect, change the theme from "Ambiance" to "Adwaita".