Well I'm making some 350 meg scans (actualy 700meg 48 bit) right now, even at 100% they hold up well, 1DsmkII up-sized with software don't come close. Even scanning a section at 8000dpi looks better than it has a right to, any softness is created by the taking lens and just gets enlarged.
I can believe this is generally true. I have a number of 35mm slides and negatives which I tend to rescan every time I get a higher resolving scanner.
The first scans were done by Kodak with their PhotoCD system at 2000 ppi. I later bought my first scanner, the Nikon LS 2000 which scans at 2700 ppi and noticed marginally sharper results allowing me to make larger, better looking prints.
My next scanner had an optical resolution of 4000 ppi and was able to deliver even more detail, or at least better defined detail from those slides and negatives that were particularly sharp.
My current scanner is the Dimage Elite 5400 II which, at 5400 ppi, produces the best result of all. I have no hesitation in making 22"x33" prints from the sharper of the negatives and slides in my archives.
This is what I believe is happening and once again the explanation lies in the MTF response of the scanner lens.
A scanner resolution of 2000 ppi equates to 40 lp/mm; 2700 ppi equates to 54 lp/mm; 4000 ppi to 80 lp/mm and 5400 ppi to 108 lp/mm.
The problem is that no lens can deliver such resolution at 100% MTF, so in practice the resolution limits are significantly lower. I'd say they are more like 30 lp/mm, 40 lp/mm, 60 lp/mm and 75 lp/mm.
It's possible to record 75 lp/mm on 35mm film, especially if the film is fine grained like T-Max 100, the target contrasty and the shutter speed fast enough etc., but it's probably not possible to capture such resolution in a scan of the film unless the scanner is theoretically
capable of a much greater resolution than 75 lp/mm.