Screw ripple.
I think our best bet is colored coins and blockchain transactions. Current exchanges will become gateways, will take fiat and issue fiat-denominated colored coins. So there will be "MtGox USD", "btc-e USD" etc. which you can atomically trade for BTC.
Order matching is another thing and IMO must be handled by separate network of scalable servers for small fee (may be, fixed per-order fee).