@rhino13: browsers that implement 'video' tags already support GPU acceleration - it is not as complex to support than it is for Flash.
@razor512: there are no HTML5 widely known games yet, as the 'majority' browser didn't support HTML5 - but there already are HTML 5 games, and they'll only become more common.
Postprocessing on videos is possible in HTML 5, it's just not really done yet. Postprocessing of Flash video is difficult, as videos are in YUV format, but Flash can only process RGB video - so the applet needs to send the frame to be uncompressed to the card, have it processed, converted to RGB and then get it back and work from that. That's the reason, BTW, why Flash on Linux is unaccelerated: no free API provides that yet.
@Izzycraft: that's supposed to be your login password, right?
@Clintonio: XP users won't get it. So uptake will be slow.
What I don't like about this IE 9, is that there is STILL no support for DOM 2 events - a spec dating back to november 2000, implemented by all other browsers...