I ran into a problem with my dashboard widgets on my Mac after upgrading to Leopard. The widgets would show up just fine on the dashboard, but they wouldn't respond to any of my mouse clicks. If this happens to you, here's the fix I used:

Open a Finder windows and navigate to the /Library/Preferences folder under your home folder. If you sort the list of files in this directory alphabetically, you should see a bunch of them that start with widget-. For example, widget-com.apple.widget.weather.plist is one name in my list. Delete all of these files.

Once you do that, the widgets on your dashboard should start responding to mouse clicks again.