I am using the bayesspam plugin for squirrelmail with great success when it comes to filtering incoming spam.
The problem with the implementation is that it is 3 years old and a tad buggy. The larger the data base gets, the longer it takes to login. Right now the server times out logging in.
I'd like to see some sort of bayesian spam filtering included in roundcube, in fact this single feature will drag me away from Squirrelmail despite the fact that roundcube is yet to become the fully features mail client that I know it will be. It has great potential and will follow the development with interest.
It would be nice to see some spam filter integration. A standalone filter would be the best, as it wouldn't depend on server configurations. Let's just see what they'll make of it :)
Yes I hope that this get into roundcube sometime.