The current crisis of the US auto industry and market — along with continued moderate gas prices and general economic doldrums — may impair the general market appeal of plug-in hybrid electric vehicles (PHEVs), at least for a while. Reuters reports:  "the introduction of PHEVs won't be the instant revolution of autos that many had hoped for. In all likelihood PHEV sales won't take off in meaningful numbers until 2012-2013."
But some aspects of this market are pulling into high gear — including hybrid diesel schoolbuses. Schools in AR, CA, FL, IA, NC, NY, PA, SC, TX, VA, VT, WA, and other states have ordered PHEV school buses through federally supported test projects. More info. 
SmallCapWorld recently offered a good overview  of the players in the hybrid bus market, including school buses.
In April, the US DOE awarded Navistar $10 million  to help create a PHEV school bus. On May 19, Daimler AG (parent company of Thomas Built Buses, a major US school bus manufacturer), purchased a 10% stake  in electric carmaker Tesla Motors. Daimler Press. 
Thomas Built is testing a hybrid school bus model. School Transportation News reports: "The experience gained from the hybrid delivery vans (which have an operating profile similar to a school bus) is being leveraged by Thomas for the school bus market. In those vans, tests revealed a fuel savings of 300 to 450 gallons per year, based on annual mileage of 10,000 miles. Assuming a $3 per gallon fuel price, the annual cost savings is between $900 and $1,350 per year." Release.  Contact: 336-889-4871.
During the research and testing phase, hybrid school buses cost about twice as much as regular school buses — but federal and state incentives through test programs vastly reduce the cost to school systems. PHEV school buses also can help schools meet increasingly strict emissions requirements, especially for idling/operating near schools.
Are any schools in your area using hybrid buses? State school transportation directors list.