Well, one data point: "38% of homeless survey respondents reported that they first became homeless outside of San Francisco or were relative newcomers, having lived in the City for three months or less." And that's actually a significant decrease from the 2007 survey.<p><a href="http://www.sfgov.org/site/uploadedfiles/lhcb/homeless_count/SF2009HomelessCountFINALReport.pdf" rel="nofollow">http://www.sfgov.org/site/uploadedfiles/lhcb/homeless_count/...</a>