I was quite surprised to try a few different searches on Google and find that there does not appear to be any regular "competition" for the "best" spam filter. This seems like a good way to make sure we're making progress on filtering based solutions.
What got me thinking about this was the enormous amount of image based spam that I get these days because it has no content for SpamAssassin to work with other than the headers, and apparently they aren't always suspicious enough to be flagged.
It seems like a simple contest to run once gone to the trouble of compiling a training and competition corpus. I know from talking with J that contests are definitely used for "information retrieval" type research. Plus it seems like it would be a breeze to get a few corporate sponsors. I'll have to ask around and see what people think.