>how many megapixel it would take to duplicate the >resolution of a modern 35 mm film camera?
Film camera has no resolution at all. Resolution depends on the lens and on the film. Since DSLRs use the same lenses, your question is in fact about comparative resolution of film and sensor. Films are different, and differently used.
24 MP and 14 bit color of D3X is more or less the same as professional color print film if used with Nikkor professional zooms (if film images are printed directly, with an enlarger).
To bypass quality of slides shot with Leica and scanned with a VERY expensive drum scanner at 20'000 dpi (resulting in 500 MP images), you need about 60 MP sensor with full 16 bit color and very high dynamic range. Why you need 10 times more MP scanning film than shooting directly on digital sensor? Because film image consists of irregular grains, eye does not see each of them, details are reconstructed in the brain from a mosaic of grains, so we have to produce an image of every grain on the secondary (scanned) image to deliver the same details, while on a primary (DSLR) image we can record the details themselves.
Middle format slides may be even better: no MF glass has linear resolving power like Leiz glass in lines per millimeter, but total amount of details captured with a bigger frame is higher, and these details are bigger and consist of bigger group of grains without need to enlarge this bigger frame that much as a smaller one, so you need a more affordable technology to scan MF slides (like Imacon). Big format slides can be even better, and can be scanned adequately even flatbed (something like Epson 750 is enough).