Have you watched the last MSDN TV episode with Richard Turner? It's a short but instructive view, I suggest you to check it out.
During the speech Rich draws a simple picture, which I believe deserves some meditation. Below an approximate reproduction.
The red line represents a service boundary, and all the black things... all the things which live inside those boundaries. Objects, components, data, etc etc.That is nothing strange, we have all digested the autonomy tenet so we should expect to have things which are at exclusive disposal of the service (hence inside its boundaries). This view, perfectly accurate, nas to be reconciled by all those diagrams where a service is depicted as a simple box or it is identified just by the corresponding web service. Would not be correct to say that the web service just sits on the service boundary, rather than being the boundary? Topologicly-wise, the two things may seem the same (after all, the only way to reach those resources IS (or should be) the webservice) but concept-wise I prefer to stick with the sit option: it helps avoiding to fall in the trap (component:COM+package=service:WebService), which could lead to all sorts of antipattern.