It is probably Unity and not GNOME Shell. GNOME Shell is available, but not
installed by default.

GNOME Shell is not compatible with standard GNOME and it is a new GNOME
backend with a java-based front end. It is just being released as alpha and
should come out around the time Natty is released. Natty will ship with
Unity and either Compiz or Metacity as window manager, depending on what
your graphics card can handle. It's defining feature is a ribbon style
launcher down the left side and a panel with a global menu across the top.
GNOME Shell has similar features, but they look and work differently. GNOME
Shell does not use Compiz and Compiz does not work with it.