I posted an article on WIT UI extensiblity at http://blogs.msdn.com/narend/articles/673183.aspx (Actually a while back, but just realized it won't show up in blog or search engine until I link it). This document was written by Tony Edwards, who previously owned Work Item Tracking VSIP client.
This is detailed and explains every extensiblity aspect of UI client, especially DocumentService, eventing model and lock tokens with plenty of examples. There are many creative possiblities for its use and am wondering what WIT addins & vsip plugins uses will come up with.
This was supposed to make it as part of VSSDK but didn't.