Not really. Some issues: eligibility, trade-in value of your old clunker (maybe more than $3,500 or $4,500), also your clunker may worth more than its face value (consider the insurance cost and property tax of a new car).
MyMoneyBlog has did some analysis on his/her own (used) vehicles; and Edmunds.com has a comprehensive article including a FAQ.
What is cash for clunker? Basically this is a bill the congress is working on to stimulate the car/truck sales, the government is offering $3,500 or $4,500 vouchers to consumers who are willing to give up their old clunkers (gas guzzlers) and apply the voucher to buy a new fuel efficient vehicle.
Good idea and a stimulus for car makers, but not necessary a windfall for consumer. The same idea has been used in Germany and it has stimulated car sales quite a bit.