Hmm, I have worked with an ecommerce site that was badly in need of spinning, but the problem with that kind of thing, is that the 'files' for the site are in a database, rather than actual physical files.
This would mean that you would need to design this software around each specific type of ecommerce site software because spinning and databases do not mix.
Another option would be to gather all of the descriptions and spin them out and then put them back into the site.
I like the idea of spinning whole sites and directories though, whatever program would need to be able to output the files as HTML files.