I've been very interested in Ripple for some time because it allows any account to issue tokens to any other account and those tokens to be exchanged for ripples or other tokens on the chain.
As I understand it, such a system would have to be built using contracts and some have already, such as GavCoin, PRoDOUG Sovereigns. There's also a distributed exchange, the name of which I cannot recall.
Has some standard been established in the community? The Ripple naming convention is very convenient; BTC/Justcoin means an IOU of BTC issued by Justcoin. BTC/Bitstamp is one issued by Bitstamp.
We're looking to issue such tokens from Justcoin as soon as possible, allowing users to trade IOUs of both digital currencies (BTC, XRP, LTC, ...) and possibly fiat on the Ethereum chain.