I can easily enough do this comparison if the local Nikon showroom has a D3 or D700. However, I think the exercise for any of us is purely academic, since I'm not in the market for a Nikon DSLR and you're probably not in the market for a '70s era film camera that weighs as much as a panzer tank. (When Slim accused me of just being nostalgic, he was basically right when it comes to the Leicaflex. In any case, I can use the lenses for it if I get the digital R10 when it finally comes out.)
The brightness of the Leicaflex viewfinder was achieved by dispensing with the conventional fresnel lens, using a full glass lens instead, and substituting a very fine microprism field for the groundglass field around the central focusing spot. The only thing I've seen come close for brightness is dropping an H focusing screen (full field microprism) into my Nikon F2, but the performance doesn't compare if you try to preview dof.
Anyway, this is all just for whatever it's worth at this point.