This is not the law yet, but a proposed law worth checking out...
Clark Howard.com states "Washington, D.C. is set to take a cue from Europe in adopting its own version of a "cash for guzzlers" program. The central idea is to allow people to take an old, nearly worthless car and receive a cash bounty of up to $4,500 toward buying a new fuel-efficient vehicle. Under the proposed system, the full amount of the voucher would phase in based on the miles per gallon your next car purchase gets. Say you start from a baseline of 18 mpg on your current vehicle and get a new car that gets 22 mpg. Then you'd get a $3,500 voucher. You're only eligible for the full $4,500 voucher if your next purchase gives you at least 10 mpg more than your old vehicle. There will be a separate program for SUVs (you'll need a 5 mpg increase to qualify); one for light-duty trucks (2 mpg increase); and one for work trucks. The latter is a real reversal in policy; small business was previously given tax subsidies to buy the most fuel-inefficient trucks it could!"