Your comment is correct, except that I believe the actual change in effective focal length is about 1.4x: "If I put a 100mm lens on a digital body, I have heard that this would change the effective focal length to 150mm (1.5x)."

In a way it's not unlike switchig to different film formats. A 100mm 35mm lens does not have the same field of view as a 100mm lens in 645 format. Since you can't use a 35mm lens on a 645 camera is really doesn't matter though.

This is a mixed blessing for those that are switching from film to digital. It's really great for people who like to use longer focal length lenses. But you still end up having to switch around your lens inventory a bit in order to get the same coverage for digital as you had with film.