[Note: I just posted this one on Barry's Blog, but thought it was worth putting here as well, with a bit of elaboration and nice graphics.]
Ed Leamer of the UCLA Anderson Forecast put together a pretty expansive and detailed analysis of the impact of the real estate business on the economy.
His conclusion, in a nutshell, is that housing volume delines are an almost-foolproof predictor of recessions. Every recession (even the great depression) was predicted by a downturn in housing volumes. The reasoning is simple: housing volumes drive jobs in construction, financing, sales and everything else, thus the volumes have a real multiplier effect while prices don't.
The entire paper is available at the UCLA Anderson Website
(note this is a 73 page pdf, but is well-worth skimming through)
If you read through and look at all the charts and numbers, another thing will become obvious though. Volumes lead prices significantly. Look at the chart on page 26 of his paper: In the current downturn (in Los Angeles anyway) volumes peaked in March 2004, but prices continued climbing through the end of 2006. In the last downturn, the bottom in volumes occured in March 1991, but prices didn't bottom until December 2006, more than a five year lag!

Click on the chart for a larger image
Chart from Leamer paper cited above, page 26.
Historically this is typical. Real estate bottoms are long and drawn out affairs. For those who don't have a 10 year or longer time horizon they may effectively become "L" bottoms rather than the very wide "U's" that they really are. So, while it is entirely possible that the worst of the volume declines are behind us and the NAHB will start moving inventory again soon, those who are looking for a rapid bounceback in prices are likely to be disappointed.
Unfortunately, those pesky mortgage backed securities don't get moved by real estate volumes. The prices of the underlying assets and the owners' ability to pay are what drives their value or lack thereof. A recovery in volumes that saves some of the homebuilders may not do the same for the owners of those toxic securities.
Worth noting that on page 46 of his paper, Leamer gives the Fed an "F" for performance since 2000! On page 53-54 he gives a nice bit of prescriptive advice that would have avoided this mess if it were followed.
Incidentally, Leamer presented this at the Anderson School on the same weekend that Michael "I will write some loans" Perry gave his now-infamous "people don't need to save" advice.
-btc