After some searching, I found this hint, but didn't find anything similar on MacOSXHints. So here it is, with thanks to the original poster who found it. Launch Terminal and then enter the following commands...
To turn Dashboard off:
To turn Dashboard on:
defaults write com.apple.dashboard mcx-disabled -boolean YES
You have to restart the Dock after making either change for it to take effect:
defaults write com.apple.dashboard mcx-disabled -boolean NO
[robg adds: I'm thinking this might be a pretty popular hint :). I tested the commands, and they definitely work. When I ran ps ax | grep Dash with Dashboard active, my three open widgets showed up in the process list. After changing the defaults, F12 no longer worked, and the same ps command didn't find any matches for Dashboard (OK, technically, if found the grep, but that's because I'm too lazy to add it as an ignored match). When I re-enabled Dashboard, I was somewhat surprised to find that my three open widgets were indeed still open.]