http://blogs.gnome.org/alexl/2011/03/22/multimonitor-support-in-gnome-shell/
In most typical cases the external monitor is used to show something static, like always visible information, a presentation on a projector, or a movie on a TV.
I very much disagree with this assessment. The only way I can work effectively in my job is to have the extra space and be able to segment tasks and applications on different workspaces and be able to know that when a switch to a workspace that the entire desktop is how I last left it on that workspace.
Anyways, thanks to a comment in the discussion for this post we have a fix.
http://blogs.gnome.org/alexl/2011/03/22/multimonitor-support-in-gnome-shell/comment-page-1/#comment-1374
Ok guys, here a hint to deactivate the stickyness of the secondary monitor (works for me on natty):
* start gconf-editor
* open /desktop/gnome/shell/windows
* deactivate workspaces_only_on_primary
Unfortunately gnome-shell is still rather unusable for getting work done. I use XFCE as my primary Desktop Environment now...but still install gedit and nautilus because they are better tools than the XFCE alternatives.