One week and many log-offs and log-ons later, this problem still persists. I found an imperfect workaround by going to System Prefs > Control Panel and selecting "Show dashboard as a space". Now I can at least bring it up on the keyboard with ctrl-left arrow.
However, this is an imperfect solution. I prefer layering the dashboard over my home screen and I miss the ability to just hit a single key to get it. It's also annoying to have a shortcut specified in System Prefs which the computer ignores for some reason. If anyone knows how to force it to recognize this choice, I'd be much obliged.
It's silly of Apple that even though there are interfaces to change the keyboard shortcut for both Launchpad and Dashboard, that they are ignored. There is, however, a way to work around this:
Use the free KeyRemap4MacBook. When you download it, open its preferences and then search for the word 'Launchpad' and you'll see the option 'Launchpad to Dashboard'. Put a checkmark beside that option and you're done
Apple should fix this, so that settings in the System Preferences > Keyboard Shortcuts, or System Preferences > Mission Control are honored.