I actually came out ahead on a scam like that. I bought a used 2003 350Z with ~50,000 miles in the fall of 2006 for $15,000. The next summer the dealer I had service done at called and said they would give me $15,500 in trade on a 2007 Z.
I realize it didn't really come out to a gain after TTL, but it was good enough reason to get me to jump on a new car.