I found the following link in macosxhints describing how to disable (or enable) Dashboard in Mac OS X Tiger.

Launch Terminal and then enter the following commands…

To turn Dashboard off:

defaults write com.apple.dashboard mcx-disabled -boolean YES

To turn Dashboard on:

defaults write com.apple.dashboard mcx-disabled -boolean NO

You have to restart the Dock after making either change for it to take effect:

killall Dock