What does strike me as a hint (perhaps obvious to some/many of you) is the fact that these modifier keys can also be added to the Active Screen Corners section of the Exposé tab on that panel. Click the top left menu in that section, for instance, and then hold down the Command key, and you'll see that each option is displayed with the active modifier key(s). I rarely use the screensaver, but there are times when I'd like to activate it, but never accidentally due to a mouse drag. So I've now got it set to activate only when I have the Command key down when I drag the mouse to the top left corner of the screen. Perfect.
Unlike the panels for Exposé, Dashboard, and Spaces, there's no explanatory text for the Active Screen Corners section that would let you think this possible, but it seems to work just fine.

