Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Do you use the Compiz Desktop Cube plugin? I don't use it so I didn't notice this but it seems you can't enable the Desktop Cube in Ubuntu 11.04 if you use Unity - a bug about this has been submitted but it was marked as "opinion" so it doesn't look like it's going to be fixed, at least not in Ubuntu 11.04:
If you try to load the Desktop cube plugin, it asks you if you want to disable the Desktop wall plugin. If you say yes, it wants to disable the Unity plugin (which I'm guessing is a bad idea on a Unity desktop).