I think… for what it’s worth… the difference is what you actually do, or what for what you’ve produced designs for. I have always thought that Product Design is a facet of Industrial Design. In that PD is on the same level with being a furniture designer, or some other name. My degree is in ID, I work with other IDer’s who design products. I think the name in more or less interchangable, and I, please see that I, wouldn’t be deterred either one way or the other… so I think it’s more semantics and language.
Which path uses what skills… that’s how you use them and what you use them for.