Gnome Shell on 11.04 is a non-starter on my desktop/laptop without adding an extra PPA...
That was from my desktop about 5 minutes ago with a fully updated natty system.
user@host~$ sudo apt-get install gnome3-session
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
gnome3-session : Depends: gnome-shell but it is not installable
E: Broken packages
If you want to get Gnome Shell working, check out this page:
They've got a PPA which can get the required packages installed.