I'd write one up myself, only my XML-fu is probably not yet up to it.
Hmm, I'll have to drag out my XML book and see if I can figure one out.
I've started plotting such a format out. At this stage, it'll look vaguely like RSS 1.0 (or perhaps 2.0); though whether it'll be compatible with RSS aggregators is another matter.
Disclaimer: I am not an XML language lawyer.
if u find one... please tell me.