I have a current bounty on this question of Noam Nisan. Unfortuantely, this question appears to be the Bermuda Triangle for Bounties, as mine is the third bounty being offered which seems likely to expire with no answers produced.
So -- I would be glad if someone would leave a placeholder answer so I could award the bounty, and then the answerer would turn around and offer a bounty of the same size (100 rep) to a question of their choice. Please consider doing that.