I've just posted an article on the Long Tail, Short Head and Search. Every site, intranet and enterprise search log I've analyzed fits the model of the Long Tail, with a very few very popular search terms, then tailing off very quickly to unique queries (the Long Tail), creating a Zipf curve.

The Short Head -- the few most frequently used search terms -- is the best place to start in analyzing search engine usage. My article also gives some suggestions for taking the information and using it to improve a search engine.