But a distributed exchange is a challenge because it's almost a contradiction in terms. A market place is one location that people come to meet where they can exchange, when that location is distributed how do you trade?
The system is physically distributed, but logically (or virtually) a unified whole. All nodes have a unified view of the ledger, and the consensus process takes much less time than with Bitcoin. The "ledger" also contains the order book. I don't see how this system could support very high-frequency trading, but I'm not sure that's actually useful. If it is, specialised exchanges could serve that market with their own privately developed order book and still be connected to the Ripple network.