And just to clarify: This means that this is not just another browser plugin like Flash, but rather allows IE9 to support WebM through the HTML5 <video> element.
The inevitable question here is how this is any better than using a browser plugin, and the answer is this avoids all the limitations of browser plugins. It opens up a whole new world of possibilities, since HTML5 video can be accessed and manipulated by the browser. A browser plugin, on the other hand, is basically a "black box" to the browser. It doesn't offer even a fraction of the flexibility you get with native HTML5 video (video support in the browser itself).
As I explained in a previous blog post on the subject, this will deal another blow to the closed H.264 codec on the desktop.
So for now, I think we can ignore the hilariously hypocritical quip about "ideology" in the Microsoft blog post, and congratulate them on doing the right thing instead!