I'm not sure this is news at all, since IE9 use Windows Media Foundation, which means that it can support any codec the user has installed.
Maybe I misunderstood something, otherwise it seems a bit dishonest of Dean to use this as an argument for their strong support of HTML5.

If they did want to support an open alternative to H.264 they would of course include the codec with the browser.
While it's great that IE users will be able to watch VP8 if they install additional software, it's not that much different from saying that they will be able to watch VP8 if they download a new browser.

While it might no be as big a deal to download a codec as switching browser, in practice this will mean that the majority will not be able to watch VP8 without upgrading their software which is something I like to generally avoid when developing a site.

So I will much rather stick with using the Flash Player where the updates is quickly spread with users. That way most users will be able to see VP8 encoded content regardless of what browser they use and what codecs they have installed.
So in effect it doesn't matter if IE can support VP8 or not. If they don't do it by default they might just as well not do it at all.