Ed Olsen, one of the nation's foremost housing experts, points out that it's much harder to stimulate housing than many people think because you have to take into account the rental market.
The primary effect of many proposals directed at the housing market would be to decrease the demand for rental units by about the same amount as they would increase the demand for owner-occupied units. This would be the effect of the proposed tax credits or loans at below-market interest rates to new homebuyers.
…The impact of preventing foreclosures on housing prices is overstated for the same reason. The overwhelming majority of families who default on their mortgages move to another unit that they do not share with others. Therefore, preventing foreclosures would have little effect on the total demand for dwelling units and hence little overall effect on market prices.
…Subprime mortgages did induce some people to buy houses beyond their means, and foreclosures would decrease the demand for the types of houses bought by these people. This would decrease the prices of similar houses. However, when they default on their mortgages, the families involved move to more modest houses or apartments, thereby increasing the demand for other types of units in other locations and the prices of units of these types. Preventing foreclosures would lead to higher prices for some properties and lower prices for others.
Read the whole thing (doc).