Ian Hickson forgot a small but important point in the pingback specification. I wrote him an email and he acknowledged my thoughts. But he does no more maintain the specification and so left it for me to pick up. I've set up the new spec pingback v1.01 containing the missing point here: http://www.rorkvell.de/tech/specs/pingback.html.en
It's mainly about the fact that a pinging source may be in format rss as well as html. Originally it was meant to be html by Hickson, but some servers do only unterstand rss. So the new spec mainly clarifies exactly this point by adding more flexibility to the source format.
Siegfried (aka Unicorn)