While its clear that eventually you will be able to use HTML5 for a wide range of non-trivial production apps, that just isn't the case today. Even the W3C says HTML5 isn't ready for prime time:
Oct. 6, 2010: As an official from the World Wide Web Consortium (W3C) told reporters today, “There is already a lot of excitement for HTML5, but it’s a little too early to deploy it because we’re running into interoperability issues.” Particularly when it comes to video content, different devices and different browsers aren’t handling HTML5 consistently. “I don’t think it’s ready for production yet,” the official continued. “The real problem is can we make it work across browsers, and at the moment, that is not the case.” from Mashable
To Microsoft: Very, very poorly played. What you should have done at the PDC is assured the large base of developers working on everything from deep zoom apps to dashboards for finance that you will continue to invest in Silverlight alongside your fledgling HTML5 efforts. You can easily afford parallel efforts. Now developers who read articles such as Microsoft Shifts From Silverlight to HTML5 will look elsewhere for the technology they need today and use those platforms until HTML5 comes of age. I guarantee you just killed many large projects in the finance sector that were planned for Silverlight, and caused irreparable damage to your company's reputation in an industry that was moving increasingly in your direction for rich client development.
I suspect Microsoft will realize their blunder and do some back peddling, but they have already done significant damage to Silverlight, a perfectly good platform in which they and many other companies have invested millions of dollars, for absolutely no good reason. (queue music) Dumb-da-dumb-dumb-dumb.
Update: Bob Muglia, President of the Server and Tools Division at Microsoft, has back-peddled with a blog post that includes an apology for the confusing statements and the following clarifications: