Great question. We have dealt with this several times since the rel="author" was released, and our belief is that the rel="author" is best used when the page, website or post should be associated with a person.
A businesses that offers a personalized service like attorneys, doctors, dentists, web designers, etc. are perfect for the author tag because it gives a face to the service, and of course, like you said, blog posts are a great use for the author tag.
Even though we have seen an increase in the click though rate for the sites that we have implemented the author tag, we do not not use it for product websites or pages, just because we feel an author tag does not give a good representation of the actual page/product.
The only exception I can think of is if the product is closely associated and sold by a person like "The George Forman Grill," but we haven't come across a client like that yet.
I hope this help and would love to hear what other people think.