A house Fairburn, Georgia in an undated photo. Sales of previously owned homes slipped 0.8 percent in December and took their biggest tumble in 17 years for all of 2006, leaving in doubt whether the worst of a housing slump has passed. (File/Reuters)Reuters - Sales of previously owned U.S. homes slipped 0.8 percent in December and took their biggest tumble in 17 years for all of 2006, leaving in doubt whether the worst of a housing slump has passed.