Given the concerns Microsoft is voicing (and they aren't the first to voice them) are well below the application itself, I tend to trust Microsoft more on this one.
Then there's XNA in Silverlight. If they believe in the security of that, why not build WebGL on top of it? Probably because they're in direct competition with one another, and Microsoft wants Silverlight to win.
I'm pretty sure you could build a WebGL API on top of Silverlight by yourself. It would suck, but it is possible.
That's why I always thought Silverlight is pretty cool as a technology, as compared to Flash. Among other things, a Silverlight plugin could also allow you to serve OGG Theora videos to users.
We cannot, however, discount the incentive Microsoft has in preventing the formation of another standard it can't control. I would consider any info coming from Redmond on this issue to be somewhat exaggerated.
And Windows-specific.
http://twitter.com/#!/paul_irish/status/81492337108328448 @AlexGraul i think chrome's record in pwn2own is a good indicator of our commitment to security while delivering great features. :)
I do know Chrome and FF just fixed a timing attack vector where you could apparently intuit the content of a crossdomain image by interpreting what hues were based on it via the application of shaders. Which means hypothetically you could read text. Like a crazy-person's OCR.