The incentive program worked - there are now 423,180 hybrid vehicles registered in CA. And the $8 yellow decals added $4,000 to the value of the vehicle. It even enabled until last July a free ride over the $4 toll Bay Bridge.
For those solo motorists who wish to continue the 'carpool privilege', they can drive a zero-emission vehicle, e.g. Nissan Leaf with the white sticker, or come January, a plug-in hybrid electric vehicle, e.g. Chevy Volt, with the green sticker. Of course, they could always carpool or take the bus and not worry about getting a sticker which are suppose to expire in 2015 - so they say.
From SF Chronicle: John Burton defends Prius carpool-lane access: "The yellow-sticker program isn't over yet, and John Burton (the California Democratic Party chairman and former state Senate leader) is a crafty guy who knows more about the Capitol than anyone around. Don't be surprised if July 1 arrives, and Burton still is zipping from San Francisco to Sacramento in the carpool lane."