My "Show Desktop" no longer functions
Tried trashing user pref com.apple.symbolichotkeys.plist, that did no good. Finally used a scattergun approach which lost all the shortcuts in my Dock, but that's no big deal, it's easily repopulated. Here are the ~Library/Preferences that I trashed:
com.apple.dashboard.plist
com.apple.dashboard.plist.lockfile
com.apple.desktop.plist
com.apple.desktop.plist.lockfile
com.apple.dock.plist
com.apple.dock.plist.lockfile

Replacing the com.apple.symbolichotkeys.plist may or may not be necessary as well. Then reboot, or log out and back in. Voila- everything reset to defaults, and it works for the first time in months.