Set sleep button to start screensaver

This will show you set the sleep key on the keyboard to start the screensaver

Turn of the sleep key

This may have to be done via gconf-editor, rather than the command line

gconftool-2 --set /apps/gnome-power-manager/buttons/suspend --type string "nothing"

Now create the custom script

System -> Preferences -> Keyboard shortcuts
Add ->
Name: Screensaver
Command: gnome-screensaver-command --activate
Key/Shortcut: XF86Sleep

Or you can turn off the monitor, rather than activate the screensaver:

System -> Preferences -> Keyboard shortcuts
Add ->
Name: Screensaver
Command: xset dpms force off
Key/Shortcut: XF86Sleep