Wow, Dennis, that is fantastic.
Having looked at the animation a few times it seems to me as though the white spot is much higher in altitude than the background bands and Andrew's black spot considerably more so. This conclusion is drawn merely from the perspective I perceive and could well be (and most probably is) faulty.
I then thought "why cannot I see a trail of debris?". The NASA IR images at
http://www.nasa.gov/topics/solarsyst...-20090720.html might answer that question.
Whatever caused the effect must have been a massive event.