I did a search for exactly that "how many angels will fit on the head of a pin?" and this is the answer I got.
The quotes are specifically on Adsense but it does not take a massive leap to see how these things could easily be applied to Google Web search.
In May 2002 they issued a press release on some new taxonomies being introduced.
I found an interesting web page that put two and two together here is a quote from it.
If you turn that around then using the same approach you can serve the most relevant SERPs for a given search term. Add in an analysis of pages that you link to and that link to you and perhaps we have our answer.
Co-incidence - the Google directory tab results are exactly the same as the web search results.
Co-incidence – results are now returned much quicker. Could this indicate a predetermined categorization?
It would not take a massive brain to decide to add one last category to a Google taxonomy tree and that is spam. So what is spam. That’s up to Google to decide. Applied Semantics had/has an Autocategorizer product that allowed the user to specify hierarchies.
Most of the press releases and papers that you find on Applied Semantics Web site by a Google search for ontology semantics taxonomy and similar searches have been pulled from the site but are available in the Google Cache.