If you have recently updated to Ubuntu 12.04 with Gnome as your desktop and are using dual monitors, you have probably noticed that only one of the screens switch workspaces and the other stays fixed. Before, you were able to fix this by using the gconf-editor, however now when you make changes to that, it is no longer working and the changes are not being applied. To fix the monitors when that does not work, you need to run the following command, which will disable workspaces-only-on-primary, which is basically doing the same thing as is done in the gconf-editor.
Run this command as the user that you are logged in as (not using sudo or as root in the command line)
gsettings set org.gnome.shell.overrides workspaces-only-on-primary false
You should not receive any output when you run the command and the changes should be applied immediately without needing to log out. You will now be able to change workspaces correctly on both monitors.