I agree with @tophee here.
On community.humanetech.com we are currently discussing strategies for reorganizing the forum and as a result we have lotsa PM communication among a network of active members and staff. Also this way of working will continue after reorg is done. Our forum is the raw body of work from which to derive follow-up actions and projects.
Notifying participants to stay within the topic and create a new PM title when they broach a new subject goes a long way, but as with email users still frequently forget to do this. And private messages are harder to find and search, so there is risk of valuable info becoming lost, fall from the radar.
In our case the workaround @tophee suggests would mean that strategies under preparation would be visible for a short time to people who should not see them yet. Maybe they even receive a mail notification from them?
Would be great if PM topics could be moderated in same way as public topics, so: 1. Select post, 2. Move to new PM topic