Will Unity be the only interface for 11.04?

Solution 1:

As things stand today, every foreseeable Ubuntu release until the end of the GNOME 3.x series will feature GNOME 3.x, minus GNOME Shell. GNOME Shell is just one part of the GNOME stack.

This is GNOME Shell:

http://live.gnome.org/GnomeShell/

This is GNOME 3:

http://live.gnome.org/ThreePointZero

Solution 2:

As in Ubuntu 10.10 (and 10.04), GNOME Shell will be available to install, and should be a first-class citizen. The existing “GNOME 3” session you can select from the login screen will remain, and will continue to provide a close to standard upstream GNOME Shell experience.

So, while Unity will be the default experience, it will remain easy to use GNOME Shell (or the existing traditional 2D desktop) instead.