I'll note that there probably aren't too many takers for this, and by my math, you HAVE to offer the $4.5k to get many people to take it. The limitations really restrict it - there's a lot of choices out there that won't net the full money.
As with many such programs, it's often not enough to change an uneconomical decision into a new one. IE if you weren't already considering a new vehicle because your old one is dying/no longer meets needs, you're probably not a candidate for this.
The Feds are basing the program off of a similar German program (Abwrackprämie) that gives 2.5k Euros (~3.5k USD) towards the purchase of a new car for scrapping an old car. The Abwrackprämie has been a huge success.* Instead of the 20-40% decreases in car sales the rest of the world has been seeing, the German auto market is on track for a record year. The Feds figure they can replicate that level of success in America with a bit more incentives and push for a bit of 'greenness' while they're at it.
* Law of Unintended Consequences aside. In all probability, German market car sales are going to drop through the floor in 2010 when this program ends with nasty results for German car makers.