So...I have it on good authority that http://virtual-notary.org/ will soon be adding Mt Gox USD/BTC to the list of exchange rates they "notarize".  Their hash chaining method (which they also publish to the blockchain every 24 hours) is sufficient to defend against rewriting of historical data, and by attaching the Cornell name to the project, they incur a nontrivial "reputation bond" against insider manipulation of data (though of course no one is immune to a breach).  What interesting things can be built with a comparatively trustworthy 3rd-party source of exchange rate data?