Paul started the effort to build an open source trade engine and had buttercoin.com lying around. We joined up with him a couple of days after he announced the idea and started working on it. Cedric and I already had an interview lined up for YC, but we all thought it was a good enough idea to add Paul and change the idea we were applying with.
Unfortunately things didn't work out and the original open source effort died off when we decided to start with a fresh implementation. We just recently release the engine we're using (with some minor tweaks) under an MIT license here: https://github.com/buttercoin/engine. We'll be backporting a bit more functionality until we can actually run it in production.
Paul left after a couple of months and hasn't been involved with the project for about a year and a half. None of the design or code is based on the original open source effort which he started. We're not trying to hide history, he just doesn't want people thinking he's associated with it anymore.
Unfortunately things didn't work out and the original open source effort died off when we decided to start with a fresh implementation. We just recently release the engine we're using (with some minor tweaks) under an MIT license here: https://github.com/buttercoin/engine. We'll be backporting a bit more functionality until we can actually run it in production.