A bug submitted @ Launchpad points out that Gnome Shell might not be available in the official Ubuntu 11.04 Natty Narwhal repositories.
Gnome Shell has been available in the official Ubuntu repositories for the past three releases, however some recent changes in Natty broke Gnome Shell and it seems it can't be fixed, which might lead to Gnome Shell being "removed and blacklisted" in Ubuntu 11.04:
Due to the state of the GNOME3 stack in natty, we won't be able to upgrade gnome-shell to 2.91.x. In addition, due to libraries that have already changed, (Bug #685225 and Bug #677382), gnome-shell is currently broken. We also have the xulrunner-2.0 transition to consider. Due to all this, we believe it best to remove gnome-shell from natty.
The bug description (which was submited by Micah Gersten, an Ubuntu member) mentions that the current plan is to get it built in the GNOME3 PPA and reintroduce it when GNOME3 makes its way into Ubuntu.