We were talking about this at work, so I Googled it and found the same thing has happened before. In addtion to the December, '08 instace, it happened in Kansas City, 10/03; in Houston, 10/07.
The most amazing one I found on Google took place in West Bromwich, U.K., 6/08. A family had just moved into a new home; when the father inspected the attic he found 12 illegal immigrants living in the roof. He made a quick exit and called the police. The 12, including men, women, children and an infant were gone when the police arrived.