In case anyone is interested, here's what I have so far: http://www.flamewars.org/~phil/btcexchanger.git

This is incomplete. I'd hoped to finish the reference implementation and set up a test node before making this available but I've run out of time/energy for now. In fact I haven't worked on this since mid March.

The RPC mechanism and a basic web interface are working, but the backend which actually causes the Bitcoin transactions to take place is missing. I was thinking about using two separate bitcoin daemons with independent wallets for each transaction, to guarantee that the transaction history for the sent coins is isolated from that of the received coins. There should be a way to do that within a single daemon but I doubt that functionality is implemented currently.

Thoughts (and patches ) are welcome.