Oh please no. /msgs scale poorly in comparison to chatter when it comes to the load on people being asked questions. If you ask something in chatter, everyone watching has an opportunity to answer while when you /msg, only that one person can respond. This is an "Eat flaming death!" sort of idea for me. Some people already have too much of this happen and it is a problem that should not be encouraged.
And we've done group /msg boxes and they suck quite a bit.
injunjoel, we already have a system for persistent public conversations and it is nodes and replies. I see extremely little advantage to some system like "/msg" but persistent and shared compared to just posting nodes. And I see lots of advantages to the existing node system over shared /msgs.