This striking paragraph is from the excellent Calculatedrisk blog, which is filled with useful data and insight:

. . .this is a reminder that new high rise condos are not included in the new home inventory report from the Census Bureau, and are also not included in the existing home sales report from the NAR (unless they are listed). These uncounted units are concentrated in Miami, Las Vegas, San Diego and other large cities - but as these articles show, there are new condos almost everywhere.

The NAR is the National Association of Realtors. Also Alan Abelson, in Barron's, has a column about the sword of Damocles (or perhaps the crabgrass avalanche) hanging over the housing marke, in the form of foreclosures waiting to happent:

Amherst [Securities] estimates this massive overhang at seven million units. That's the equivalent of 135% of a full year's existing-home sales and chillingly greater than the 1.27 million units that made up the overhang in early 2005, when the housing bubble had just begun its dizzying and more than a little lunatic ascent.

Put another way, of the 56 million units that the Mortgage Bankers Association says make up the mortgage universe, Amherst gauges 6.94 million units are in what it dubs the "delinquency pipeline" eventually headed for liquidation. And it reckons that another 300,000 mortgages replenish that unwelcome flow every month.

Essentially, then, this shadow inventory represents a massive furtive supply of future foreclosure.

The full Abelson column is here, but i'm not sure it's available to non-subscribers. For some broader perspective, consider that Moody's and Zillow agree that about a quarter of homes are "underwater," meaning they are burdened with mortgages greater than their value