Turn off glass effect for the dock:
defaults write com.apple.dock no-glass -boolean YES; killall Dock
Turn on glass effect for the dock:
defaults write com.apple.dock no-glass -boolean NO; killall Dock
Want something else for the dock?
Most (all?) of these work by replacing the glass graphics.
(I saw a really good one on the weekend but can’t find it now.)
Update: It was inevitable that something like leoparddocks.com would appear. Enjoy!