Open Source Twitter

Dave Winer wonders about creating an open source Twitter. I've done some musings previously on the mechanics of Twitter. In short it would be interesting to create a Twitter. I'm not sure if Dave is thinking mostly of Twitter clients into the existing network, or sub-nets that may or may not converse with the existing network. Twitter is done again this morning. I'm not sure if that indicates the challenges in creating such a scalable network or in the current implementation of it.