There are several groups working on them in ways that look professional to me. "I'd be willing to invest in them" professional, I mean. And I didn't even know the one Peter R. linked to. Decentralized exchanges will, almost certainly, hit our market big time, within one, maybe 2 years. The question is if, and if so, which one of them will became the "standard" system, how much of the total (fiat) exchange pie they will take up, and what amusing, as-of-yet unforeseeable consequences this change in our btc ecosystem will have (some form of "network of escrows" will probably be unavoidable, and I'm already looking forward to the discussions and drama around how those will act)
It has been developed, and it is being testing for nxtcoin:
https://bitcointalksearch.org/topic/nxt-decentralized-asset-exchange-discussion-thread-460343
You missed my point. "It has been developed" is like saying "There is an exchange" 2 years ago. There was (almost) always more than one, and only time will tell which one will come out on top. That said, I'm not a big fan of NXT. Without getting into the semantics of whether it makes sense to say that a proof-of-stake coin is pre-mined in the sense that has been used as criticism against scammy alts, I don't like their 'founders monetization model' for the lack of a better term.