I was actually just thinking about that, after I posted this. Originally I wanted it to be an actual thread on the forum, but after thinking more about it. The pm feature will work perfect for what I am looking for.
I used a vBulletin 3.x forum the other day to send a couple of private messages... wow... I forgot how crappy they were
But I think, in a way, this is why they've been renamed conversations here. They aren't private messages because you can have loads of participants. And don't forget you can enable/disable attachments, increase the number of participants and there's loads of enhancements to be had by Conversation Essentials.