Pardon my ignorance as I try to wrap my head around Ripple, but couldn't it just use BTC instead of XRP? How would it not work as well? Is the only reason for it not working as well with BTC is that Opencoin wouldn't have a stock of XRP to hand out to promote the system? If that's the only case it doesn't sound like a good reason. If so I would assume that as soon as Ripple becomes open source a fork would be created with people opting to prefer their "free-ish" transaction fee be denominated in BTC as opposed to XRP?