As mentioned by Desertdog, I just read that all the "donate your car" charities are hurting. No one wants to donate their car for a tax write-off if they can get the $4500 "cash" for a new one. So now even more people too poor to buy a newer vehicle are SOL.
This also adversely affects people who don't want to get into debt. I have never made a car payment in my life. I've always paid cash. My first two vehicles were older used cars because that's all that I could afford at the time. If this program continues for any length of time, if I wanted to buy my first car in the immediate future, I'd likely have to take a loan out because of what will now essentially be artificially inflated prices on the limited choice and amount of used vehicles that will be on the market.
That or buy something with 200K on the odometer that will end up nickel and diming me, because the way this program seems to be working, it's taking all the "mid-range" less than 10 year old, under 100K, vehicles off the market. You're left with the really old clunkers and "newer" used vehicles that will be out of many people's price range.