I'm really tired of people confusing IOUs with value.

With cryptocurrency, you can move the Value online, but with fiat currencies, all OT and ripple can move is an IOU for that currency/commodity, and then you've created a central point of failure as people collect their IOUs.

A cryptocurrency P2P exchange we can make right now; overcoming the value problem is the only problem we're stuck on.

To save time, I've summed up the process (with pics!) and streamlined the criteria for a real P2P distributed exchange here: https://bitcointalk.org/index.php?topic=212841.0