You will start seeing this more going forward on cars from that era. First, the cars are just harder to come by now, naturally. Another major factor is the Gen Y/X crowds are now at a point in their life where they are raking in the cash, while the boomer generation is retiring and beginning to live on a fixed income.
Us Gen Y/Xer types grew up with SVO mustangs, Typhoons, Grand Nationals, Irocs and GLH-S anything from Chrysler. Now most of us are at a point in our lives where have the coin to buy whatever we want. I really doubt you'll see old GTOs or Mustangs appreciate at the same rate like they have historically. I guess I'll wait another 10 yrs and then list my Iroc.