Bob Atkins has a detailed article on digital cameras and lenses and whether a good lens matters or not:

Based on MY particular film and digital workflow:

• Even on a 6MP DSLR like the EOS 10D, better lenses give better results
• Digital scans of high resolution film (Kodachrome 25) at 4000dpi on a FS4000US scanner yields higher resolution images than those shot directly with an EOS 10D. Not surprising, but confirmed by experiment.
• While 10D resolution is somewhat lower than ISO 100 film, it's very little affected by ISO setting so it's quite possible that high ISO digital resolution may be better than scanned high ISO film.
• From a practical viewpoint, 10D images printed on an inkjet from digital files are probably equal film up to maybe 11x14. They are certainly good enough to be very hard to distinguish from film.
• As an aside, if you want the ultimate in sharpness do it the right way. Get a large format camera and shoot film!