I used to use a utility in gnome2. I would set it for ever hour or so, lock the screen for ten minutes. It was great; it couldn't be turned off or disabled and really helped me break away from the computer sometimes.
But now, I totally forgot how to access it. It was something in the top panel I think...I remember it would show a little bar graph in the panel; when the bar graph filled up it would lock the display.
Does this ring a bell with anyone?
edit: found this post, listing gnome-nanny, timepkr, and timeoutd. But none of those are what I'm looking for...