A difficult question to answer, since hybrids could be popular with those who don't purchase them at all (like all my friends who love my HCH and the mileage when I drive them around, but probably wouldn't buy one themselves).

Hybrids comprised 1.54% of all U.S. light duty vehicles (or 23,048 units) sold in June 2006.

http://www.greencarcongress.com/2006...es_of_hyb.html