I'm stumped... I can't change this particular shortcut either. I tried double-clicking the shortcut and holding down the keys, as Snow Leopard says to do in the Keyboard Shortcuts section of System Preferences' Keyboard pane, and all I get is beeping.
Look up in Dictionary, from where? In Safari, Cmd-D is Add Bookmark. I don't know how exclusive keyboard shortcuts are, but obviously, you can't have two commands assigned to the same shortcut in the same application. Perhaps you'd have better odds using a three-key shortcut.