I think this is a fantastic idea, but the filter could stand to benefit from better requirements. For example number of direct messages sent to the user (since people don't DM spam bots very often), number of @replies sent to the user.

It could stand to happen that a lot of users who use sites like Brightkite or other sites where they broadcast what they're listening to or where they're at could get caught u p in your spam/bot filter simply for making heavier use of such sites.

I'd revisit your filter to look at how the user interacts with other users, versus just monitoring posting behavior.