From the Link
www.cars.gov
Your vehicle must be less than 25 years old on the trade-in date
Only purchase or lease of new vehicles qualify
Generally, trade-in vehicles must get 18 or less MPG (some very large pick-up trucks and cargo vans have different requirements)
Trade-in vehicles must be registered and insured continuously for the full year preceding the trade-in
You don't need a voucher, dealers will apply a credit at purchase
Program runs through Nov 1, 2009 or when the funds are exhausted, whichever comes first.
The program requires the scrapping of your eligible trade-in vehicle, and that the dealer disclose to you an estimate of the scrap value of your trade-in. The scrap value, however minimal, will be in addition to the rebate, and not in place of the rebate.