Ray Ozzie, Chief Software Architect of Microsoft (along with Scott Guthrie, General Manager of the tools group) presented the keynote at MIX '07, 2nd annual Microsoft's Web UI and development conference
The core tenet of Ray's argument is that client side computing is vital to delivering rich experiences. I think he made that case well (and I agree with the rationale), but Ray didn't really address how or why Silverlight specifically and WPF generally was better than browser + DHTML/SVG/Flash/Java, or whatever, in terms of ANY richer function and/or end user benefit.
Specifically, the undertone of the arguments, from both Microsoft and Adobe, is that a single sourced runtime is better for the developer - more consistency across a wider variety of platforms (browsers, OS'es, devices, etc.). And, as a practical matter, its hard to disagree with that - and there's enough that's "open sourced" by the vendors to reduce impedence in the development chain.
Speaking of which (and not to be overlooked), the development chain that Microsoft is putting together is nothing short of phenomenal. If there's a "secret sauce" in Windows continued dominance in the Enterprise (and thus, everywhere else) its through the tools, class libraries, etc.; they continue to define the cutting edge of developer productivity. Perhaps somewhat counter-intuitively, it seems particularly well designed for small teams and becomes an easy way to develop prototypes, and, in turn, go from prototype to production.
Platform success, which I'll define as self-sustaining propagation and increasing barrier to exit, is definitely AADD (All About the Developers, Dummy).
There was also a Michael Arrington interview with Ray and Scott, which might as well have been conducted by a Microsoft employee. What few interesting questions were asked went basically unanswered, and at least that way there would have been less mumbling.