The Case-Shiller housing index soared to 8.1%, prompting modest celebrations for the biggest year-over-year increase in housing prices since 2006. But while home pricing and permits are up, this may only be a short-term gain. "There might even be a bubble," argues Yale economist Robert Shiller, urging caution to investors. Issues for Shiller, co-creator of the S&P housing index that bears his name, include the huge "shadow inventory" in foreclosed homes that have yet to clear through the market, as well as the fate of Fannie Mae and Freddie Mac, which languish in conservatorship. Add to this the recent drop in homes sales, and the increasing shortages of skilled labor in the construction workforce, and the outlook is murky, lumpy, and unpredictable, at best.