Google's justification doesn't really add up, and there's a strong chance that the decision will serve only to undermine the use of the <video> tag completely. This is not a move promoting the open web. If anything, it is quite the reverse.
As I see it, Google is doing Adobe a huge favour. This isn't about openness at all. It's all about proprietary stuff. Google is pushing WebM, which it owns. But this will result in more support for Flash, which Adobe owns.
This is major hypocrisy.