I just received an email about a bill before the California assembly. AB 493 may go to a vote today. The bill, sponsored by Ira Ruskin (D-Redwood City) would impose a fee or a rebate on new vehicles sold in California. A fee would apply to a vehicle that produces a lot of polluting emissions, and you would receive a rebate (based on a sliding scale) for cars that produce less. For instance, a Prius would get you a maximum rebate, while a Hummer would be slapped with a big fee.
The idea is right—having the market play a factor in cleaning the environment. However, in principle, I don't think that taxation (and yes, it's a tax when you impose a mandatory fee) is a good means to change behavior. Though, there should be some kind of penalty for excessive pollution. It's a tough issue.
This bill does not take into account that many who use trucks or large vehicles for work (e.g. yard crews, builders, etc.) would have to pay the penalty, nor does it address the fact that car companies are dragging their feet on lower emission vehicles (and probably, as a result, artificially inflating the cost of cars like the Prius). If a Prius cost as much as a two door coupe, then we'd see thousands of them on the road.