Here's what Pete had to say:
One of our customers just launched a beta of the most interesting integration of folksonomies and faceted navigation that I've seen so far. It balances the rigidity of facets built from a controlled vocabulary with the potential anarchy of raw folksonomies.
Users can iteratively refine their search using any combination of controlled vocabulary terms and user contributed tags. To see where these faceted tags come from, find a product and then select Write a Review. I plan to explore further when I have the chance, but at first glance, it looks very nicely done!