The recent activity stemming from Brad Fitzpatrick's Thoughts on the Social Graph reminds me of something I was thinking a lot about back in 2004. See especially Mark Atwood's post Five billion social network sites, each about one person on the Social Network Portability list.
The idea I had three years ago was the notion of people hosting their own data and the social networking sites merely being aggregators.
Steve Mallett started a mailing list and a website about this same idea, calling it DataLibre. Unfortunately the site no longer exists but here is an announcement of the original site: http://www.oreillynet.com/ruby/blog/2004/09/datalibre_update.html
Steve really drove the vision but I posted numerous thoughts on the idea. Because my blog didn't have categories at the time and I haven't gone back and categorized my old stuff here's a list of my relevant posts:
Brad's article (which is a lot more concrete than anything I ever said) seems to be rejuvenating discussion about this sort of thing. See, for example, the post More on social network portability on Plaxo's blog.