At the present time (Dec 2014) the BitShares/BitUSD market peg is not being enforced by the prediction market described in the link above. Price feeds are being supplied by delegates. I believe they intend to transition to such a scheme when their system is more mature.
Although the effectiveness of prediction markets is well established, the implementation being proposed by Bitshares has never been tried before. Unlike traditional markets with fixed settlement dates and deliverables, the Bitshares prediction market will run in perpetuity and deliver nothing. This is not without controversy, a sampling of which follows:
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