The Washington Post had an article dedicated to uncovering the reason that the housing market in Washington has been slow in recent months. While it runs through several possible explanations it leaves off the most obvious one: prices are too high.
Inflation adjusted house prices are more than 50 percent above their pre-bubble levels. Back in good old econ 101 we always taught that if supply exceeded demand then the price should fall. The fact that prices in DC are so far above their pre-bubble level would be good evidence that this is the problem.