Hybrids: Taken for an Enviro-Friendly Ride Newsweek Jan. 9, 2006 issue - King County, which encompasses Seattle, has a bus fleet of 1,400. Only 236 are hybrids, but fleet manager Jim Boon anticipates that number will multiply: "Having the stored energy in the batteries gives us much better fuel economy." Full Article
There are some more details on the Seattle hybrid busses in this PriusChat thread. This wasn't initially a smooth ride, and there were complaints; however, now that the "bus tunnel" is closed to turn it into a "light rail tunnel" (see the thread mentioned above), the hybrid busses should be seeing much better return on investment.