We wrote how to show desktop in Ubuntu 12.04 by enabling "hot corner" feature.
In some cases this setting will not take effect after a computer is restarted. It seems that unityshell plugin is rewritting the expo plugin configuratin so this could cause the issue.
You could work around the issue by setting expo plugin to be loaded after unityshell:
1) Start gconf-editor
2) Find /apps/compiz-1/general/screen0/options/active_plugins key
3) Double-click on the key active_plugins will display edit window
4) Place expo under unityshell
5) Click OK to save the changes