Microsoft announced during the build conference, and Steve Sinofsky reiterated in a
blog posting that: "For the web to move forward and for consumers to get the most out of touch-first browsing, the Metro style browser in Windows 8 is as HTML5-only as possible, and plug-in free. The experience that plug-ins provide today is not a good match with Metro style browsing and the modern HTML5 web." Sinfosky goes on explain why Microsoft will not include Flash and why it's no longer needed. It's as close as we'll get to an obituary for Flash.
Update from Thom: Added a note in the 'read more'!