I've heard cash-for-clunkers blamed for it (and I'm sure that contributed, albeit slightly), but the reality (IMO) is that there's just a general shortage on used cars because people are keeping vehicles longer in uncertain economic times.

Add in the general public's assumption that a 1-2yo car will be a better value, and prices for used cars are at irrational levels. My mom traded in her 2000 Honda Accord late last year. The car had 104k mi and a transmission that slipped/grinded into every gear - CarMax offered her $1k over kbb.