There is a new study out on the U.S. housing bubble. A precis:
… [it] shows 39 percent of all single family housing valuation in America is extremely overvalued and at risk for a price correction. The number of markets deemed “extremely overvalued” increased by 11 percent to 71 metro areas in the first quarter of 2006, up from 64 metro areas in the fourth quarter of 2005. The most overvalued markets continued to show the strongest price appreciation, while the most undervalued continued to experience the weakest price appreciation.
More here, including some detailed data.