The most accurate #'s are found on the individual college websites, either by searching the Common Data Sets or the admissons websites. Stanford's 2007 student profile lists 20 transfer students accepted out of 1400 applicants=1.6%.
Undergraduate Admission | Class Profile - Fall 2006
And trust me, if you click on the link, it will take you to the stats for incoming students for Fall 2007. Scroll down to find the transfer info.