I created a similar product to Delicious Library years before its launch and peer to peer version of it was the direction I had been planning to take it. This was all long before mobile devices, long before Apple SDK's had any support for syncing such data, etc but the idea was there and wasn't just mine, I had users requesting this as well.

But alas the revenue dried up with Delicious Library hitting the scene. It received crazy levels of press about how beautiful it was. I recall feeling at the time like they were paying the press to cover their product but knowing what I know now, I'm quite certain that was the case. The reviews never mentioned any competitors. There were several all at a cheaper prices with more tons more features.