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.
But Windows 8 will come with two versions of IE10. The Metro interface will be plug-in free, but the normal desktop version of IE10 will be a 64-bit version that will support plug-ins. Adobe has a beta version of Flash that will run on the developer preview of Windows 8 and IE10, so we haven't seen the end of Flash yet.
There's always a transitional stage... perhaps this is the first salvo? I do seek a trend to tighter control in browser apps in the future: too many avenues for bad actors STILL exist after 10 years of hardening applications and network infrastructure. Either/Or for a few years (or one OS Version) then Neither/Nor after that.