Everyone has thier own way of predicting the bottom. I think it depends on the house,location, and inventory
My Method of predicting the bottom, is calculated using the following method:
My recent house was built in 1993, and sold for $200k. I assume a 3% increase appreciation per year. As of January 2008 the house should be priced around $290(with no real improvements added). The houses around me are currently being advertised at a average of $400k. The homes that have sold fast(bank owned fixers) have been priced at $350k.
In my neck of the woods, the bottom will be when prices come down to $300-$330. Not sure when that will be though. But thats the target price for me.