The Ever Degrading SpamAssassin
Metafilter pointed me over the the following Google Answers post which a spammer asked for a breakdown of how SpamAssassin (A popular server side spam filtering program) really works. They offered just 200$ for the information.
Now SpamAssassin is indeed open source, so the information disclosed in the Google Answers post was not earth shattering, but when explained simply in a not overly technical way gives spammers the head start in working around it.
We use SpamAssassin here at SDF1.NET and have noticed the decline in its usefulness over the past couple of months, one can only hope that the next rev of the software will help.