I just noticed something the other day ... I have F8 set to open Dashboard, and if I just hold it down, Dashboard will only stay open while the key is pressed. This is geat for a quick time, traffic, weather, etc. check.
[robg adds: This works even with the default F12 key, of course -- unless you have a machine that lacks a dedicated eject key. On those machines, holding F12 will eject your CD/DVD. So to make use of this 'instant off' mode, you'll have to reassign the Dashboard keystroke in the Dashboard & Exposé System Preferences panel. This mode, obviously, isn't useful if you need to interact with a widget, but it's great for just eyeballing some that are already open.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050524154252310