What, the housing bubble isn't here-and-now enough for you? If prices fall and intrest rates go up even small amounts, the market could wipe out nearly everyone who bought within the last several years, especially if they're on those ubiquitous flexible-rate mortgages:

http://patrick.net/housing/crash.html
http://graphics10.nytimes.com/images/2006/08/26/weekinreview/27leon_graph2.large.gif

(will comment more later... gotta go to work)