The current draft states the content type for trackback responses should be application/xml. Perhaps TrackBack should be assigned a dedicated mime type so that external applications can be registered to receive TrackBack responses/documents.
Change the phrase "The Content-Type header for such document MUST be either 'text/xml' or 'application/xml'" in section 2.2 to read "The Content-Type header for such document MUST be 'application/trackback+xml'".
Change example HTTP traces referencing 'Content-Type: application/xml' to 'Content-Type: application/trackback+xml'.
Potential backwards compatibility issues given the mime types are so fundamental.