Never though I would say this, but I really wish somebody could create Pegcoin, all the advantages of instant, decentralized transfer of bitcoin, but pegged to fiat currency value. Obviously I prefer something with a limited supply like bitcoin, but for now it would certainly make arbitraging a lot easier... Probably not a viable idea to implement though (not to mention the legal issues).
Ripple?
I haven't really looked into Ripple decently, but isn't it a lending system?
Ripple is great, IMO. It has incredible potential as a payment system and is far more versatile than bitcoin in that regard. In that way, since you can move in and out of so many currencies and IOU issuers, Ripple could be very useful for arbitraging. Too early at this point, though.