I got some ideas on IRC, and then Mark and I sat down and came up with some random stuff that we think helps describe if someone is active or not (like number of posts, comments, time since last post, etc.). It'll be in the bug when I upload the patch (when I finish it). It's definitely not as good as it could be...luckily, it's easily changed so if we come up with better ways of measuring activity later, it can be changed.