Interesting points! We are using a naive Bayes classifier right now, and we're working with some social aspects, especially as they relate to Latent Semantic Analysis. We've considered doing a mini Netflix Prize for further improving our algorithm :-).

We've also done a lot of tweaking to get the current filter as smart as possible (thanks PG for your thoughts in A Plan for Spam!) Tweaked things like the number of interesting words to count, weighting of title vs body, etc.

We'd love to talk with some machine learning experts, drop me a note if you are one!