Ubuntu 12.04 suffers from a regression bug in Compiz, when activating the scale plugin with the key combination Super+W only the windows of the current workspace are shown. The released fix has been taken back because of troubles implementing the fix without breaking any shortcuts.
Here is a PPA containing the fix. Execute the following command consecutively.
sudo apt-add-repository ppa:timo-jyrinki/compiz-12.04-fix-933776 sudo apt-get update sudo apt-get install compiz
After a restart Super+W should bring up all windows from all workspaces.