
"Fundamentals" curve assumes more or less unchanged interest rates and steady wage inflation that averages 3%/year going forward. For the top tier I use a soft-landing scenario (exponential decay to fundamentals). To extrapolate two other tiers, I use quadratic approximations of season-adjusted HPI points from the last 9 months.
Summary of predictions:
* Aggregate Case-Shiller bottom in February '09 in 150-155 range (late '02 pricing)
* Top tier: 5% decline followed by many years of scraping along the bottom
* Middle tier: 6-8% decline followed by a mild bounce-back
Caveats:
* Bottom tier may get some support from FHA short-refinancing program, in which case the decline will not be so severe.
* I'm assuming that GSE conforming-jumbo loans and higher FHA loan limits stay with us until the return of a healthy jumbo market. Technically, they are supposed to be discontinued on Dec 31, 2008, but it's likely that loan limits will be extended.
* Interest rates may not stay the same. Here are two alternative scenarios, with rates heading to 5.5% and 7.5% long term, respectively:


I'm not a prophet and I may be seriously off, only time will tell.
UPDATE: recalculated the "fundamentals" curve using actual CPI and mortgage rate data.