Dark Exchange is the truly P2P way and it cannot be any other way without introducing banking in the system, which then makes it less P2P.
Don't be too sure about that yet... I think I've got this part cracked.
Over in this thread, I've laid it out better, but not too completely yet.
It would be regionalized, based upon Pingtimes. Each client would log on, note all other clients within X milliseconds from them, and create a board (like BTC-E's main trade engine) for that region.
Every client would create it's own board, and only those within Xms would access it. When that client logs out, that board ceases to exist.
Can't see that possibly being competitive. For starters, the lack of standard, curated and supported ways of transfering FIAT would massively hinder volume.
All these things are academically very interesting, but sadly simple beats complex most of the time. As long as "normal" exchanges continue to work, this isn't going to catch up.
I hope you prove me wrong.