I agree with this post entirely Matt, very well written.There is a difference between the ISO standard for film and the ISO standard for digital, but that probably isn't the biggest problem with using any camera (not just digital cameras) to meter for another camera.
The biggest problem is that cameras are systems themselves, and each part of the system can add variation.
For instance, if there is a marked difference between the light transmission of the lens (the "T-stop") and the aperture of the lens (the f stop) then reading the f stop will introduce variation.
If the shutter speeds vary in accuracy between the two cameras, and the reading obtained using the digital camera is adjusted or exposure, then the difference in the two shutters will introduce a variance.
The exposure systems in digital cameras are designed to give good exposure with a system that is more like slide film than negative film. The ISO standard for film is oriented toward negative film. Differences in films often require us to develop separate and distinct EIs for films that have similar ISO ratings.
It is no doubt possible to, through care and experience, develop a customized approach to making use of the information from a digital camera's meter (or any other camera's meter) to expose film in another camera, but the differences between the two systems do make it necessary to both be very careful about metering technique (such as using a standard focal length on a zoom lens) and to be very precise about using the right EI - customized to that particular combination of cameras, lenses and film.