Personally, I don't want my users to be able to PM. So if you add that function, please make it an option in the admincp, so it can be disabled site wide.
Also, I would really like it if there were a way to set two parameters (instead of just one) for the auto-pruning of messages. I would like one to control the amount of time the post is displayed for all to see, and the other to control the amount of time the post is viewable by the Admin for accountability purposes.
For example, I would personally like the posts to just be viewable on the website for an hour, after that time, the users having that converstation are gone, and in my mind there is no reason to keep broadcasting their discussion. However, to protect from abuse, I would like the system to retain all posts for a period of one week, so it doesn't become a "he said / she said" match when claims of abuse are reported. That way the Admin can go and look at exactly what was said and take the appropriate actions.
That's my 2 cents! Thanks for a great mod!

Jeff