I agree, it would be interesting to have more results than just right vs. wrong.
Exempli gratia, with Scoble, one prediction was arguably half-right (minus the stylus pen, the iPhone essentially qualifies), one that was correct at a later date (as Joe said), one that is marked correct already (RSS becoming mainstream), and one that is simply wrong (re: friendfeed).
I also agree that selection bias could skew results badly, but the idea overall is excellent.
Also, props for the disclaimer on the page! "Past performance is no guarantee of future results." Almost hofstadterian levels of indirect self-reference.
Wrong Tomorrow by Maciej Cegłowski is a very simple site for listing pundit predictions and tracking them [FAQ]. It doesn't come with prices and active betting... but a simple registry of this kind can scale much faster than a market, and right now we're in a situation where no one is bothering to track pundit predictions or report on pundit track records. Predictions are produced as simple entertainment or as simple political theater, without the slightest fear of accountability.
This site is missing some features, but it looks to me like a starting attempt at what's needed - a Wikipedia-like, user-contributed, low-barrier-to-entry database of all pundit predictions, past and present.