You'd think Adobe would notice something like THE major online video site not getting a picture with their latest software version, and that they'd delay release to fix that issue before they annoy potentially millions of people, but nooooo... It's Adobe we're talking about here. They always go the extra mile to annoy people with bugs.
It's more likely a bug at Youtubes or IE9's end rather than Adobes - the videos work fine when they're embedded at any other site despite the fact that the streamed video AND player are in fact from Youtube and not the site you're at.