What are you doing on September 8th and 9th, 2007? If you are in the San Francisco Bay area (or can get there), and more precisely Richmond, California (a bit north of Oakland and Berkeley), it appears that there will now be an "unconference" called "SocialNetworkDevCamp" with the purpose:
SocialNetworkDevCamp will focus on API and Widget development from Facebook, Twitter, Pownce, Linked In and others. The camp will also start the process of identifying open APIs and data structures which would facilitate the creation of open standards for social networking.
Very cool to see.... and hopefully it will stimulate a good bit of discussion and action around the potential mashups that can occur between all these various services. "Open standards for social networking" would also be very good to see!
If are interested in attending, just edit the wiki page and add yourself to the list of participants (or volunteers).
(Tip of the hat to Julian Bond for raising this issue in a Skype groupchat focused on mashups.)