Just in case anyone except me cares: the empirically-determined answer is 'no'.
Well, I care, as I was thinking of doing the same thing. I am trying to cut down the cable clutter on my desk and so thought a nice slim displayport cable would be preferable to the thick DVI one I am using. I am not the most IT bright person about, so are you saying it cannot be done, i.e. displayport to DVI or is 'dual mode' something different? I would appreciate your input.