Bob Muglia inadvertently caused a flurry of discussion with his focus on HTML5 at PDC. Now the dust has settled, I think everyone understands that we are still focused on silverlight as a first class target for app development, but that the increasing pace and scale of consumerisation in the enterprise (and variety of devices in the consumer space) means Microsoft (or Adobe, or Apple) can no longer guarantee that they will be able to place an agent on every end client accessing a web resource. In these cases, HTML 5 is the answer, even if pervasive adoption is still several years away.

There's a really informative history of HTML 5 on the Massachussets Technology Review, that also lays out why it is so important:

It actually shows the browser manufacturers (all of them, not just MS) recognising that problems with the web were not going to be resolved by XHTML and striking out collectively to do the right thing,