I recently posted a plugin titled PGSimpleShare to my GitHub repo. You can clone/fork it here : https://github.com/purplecabbage/phonegap-plugins/tree/master/WindowsPhone/PGSocialShare
Note the implementation of the share function is public, returns void, and expects 1 string argument, this is currently the signature for ALL plugin methods that are callable via the framework.
Hopefully this is enough for everyone to get started developing plugins for Windows Phone! Also, I have attempted to keep PGSocialShare device-agnostic in the definition of the methods, so hopefully we will see the same plugin implemented on other devices as well. I look forward to your comments, questions, and critriques!