Dr. Gord Cormac , a UW CS Prof, recently gave a presentation on comparing various spam filters (http://www.csclub.uwaterloo.ca/media/Spam%20Filters:%20Do%20they%20work%20and%20Can%20you%20prove%20it.html). While his talk is focused in an academic nature, I figured it would probably be of interest to most of the webhosts here looking at comparing different types of spam filtering software.