In Ubuntu 10.10 (and possibly previous versions), when I connected my laptop to an external source (monitor/projector) and set it not to clone my desktop the display on the external source would not show the (gnome-)panel (just a full-screen desktop with no icons).
Since upgrading to Ubuntu 11.04 (using Unity) when I now connect the external source has the panel on there. Is there any way to revert to the previous behaviour in 11.04?
You can (for now) choose "Ubuntu Classic" when logging in - and, although this won't give you Unity, it is a workaround for your problem.
(From what I've seen, Unity currently has major problems with multi-screen displays).
This is not possible (currently) in Ubuntu when using Unity as your desktop environment. However, you can install Gnome-panel and login to "Gnome Fallback Mode" to get this setup, or use XFCE or LXDE.