According to the US Census Bureau News for April 2008 (PDF), there are now 18.6 million empty houses in America.  Salon.com’s Andrew Leonard, “4.7 million are for “seasonal use” only, the Census tells us — unoccupied vacation homes, in other words. 4.1 million are for rent, 2.3 million are for sale, and the remaining 7.5 million ‘were vacant for a variety of other reasons.’ “

The census also lists the total number of homeless in America as 759,101, so there are 24 empty houses for every homeless person in America.
(via Salon.com