Thanks to CR for pointing out this BusinessWeek article with quotes from Fannie Mae's chief economist:
"The federal homebuyer tax credit shifted demand in the U.S. housing market without having a lasting impact on prices, according to Douglas Duncan, chief economist of Fannie Mae, the largest mortgage financier.
“Temporary tax credits change behavior temporarily,” Duncan said today at a National Association of Real Estate Editors conference in Austin, Texas. “It’s simply shifted demand forward...”
“It actually created some price appreciation that’s not supportable long term,” Duncan said of the tax credit."