[Wlug] How to stop screen blanking while watching movie?

Windows XP and OS X are both smart enough to not blank the screen or invoke the screensaver when I'm watching a movie. My P4m-based laptop (Dell Inspiron 8200) running debian etch blanks the screen after 15 minutes, even if I disable the screensaver and all power saving options. I don't watch movies on it that often, so I resigned myself to hitting the CTRL key every so often when I do. I installed Ubuntu Guilty Gibson on my Opteron-based desktop (Asus A8V Deluxe motherboard) yesterday and found it does exactly the same thing. Do you have any ideas how I can get them to stop blanking the screen, at least while I'm watching movies? -- Thanks! Rich