Gnome3 predefining virtual desktops
Solution 1:
Answer to this old question that was not answered, but you still get here from search engines so..
In Gnome 3, if you don't have Gnome Tweak Tools already, install it with sudo apt-get install gnome-tweak-tool
.
When you have it, start it up and go to Workspaces tab, change Dynamic to Static, and set amount of Workspaces and weither you want it on primary display only or not.
Cheers!
Solution 2:
There are a new set of extensions which let you get some of the gnome2 look and feel back. One of those lets you disable the dynamic desktops and instead lets you have a fixed no of static desktops.
You can find there here
http://intgat.tigress.co.uk/rmy/extensions/index.html
And there's a tutorial how to use the extensions.
http://www.webupd8.org/2011/05/new-gnome-shell-extensions-that-provide.html