Extend that graph a bit...
Look at the graph posted by WWW. Now, this isn't scientific OR mathematical by any stretch of the imagination, but consider this:
The graph shows 3 peaks, 3 troughs. The peaks get slowly higher, each trough (corresponding to a bad point in recent economic history) is relatively deeper and definitely wiiiider than the one preceding it.
The peaks come 1975ish, 1980ish, 1990ish, 2007ish
the troughs go up in appx 10 grand increments
So, are average house prices going to drop to around 80-90k, and is it going to take 20-ish years for the next peak to come around?
And yes, I am having a quiet day at work. And yes, I've already said it's all eyeballed rather than calculated, but if you believe in economic cycles, the you've got to wonder what's coming...