Mime-type intersection
The Comment API works by POSTing a XML document to a URI. TrackBack works in nearly the same way but POSTs data in the format of an HTML web form. The only difference between the two is the data format, i.e. their mime-types. And Aaron Swartz may have the key to mapping between the two.
Mapping between the two isn't just a corner-case of some emerging web services. If XForms takes off then this is going to be a core area of the transition from traditional web forms to XForms.