According to Microsoft, the Metro version of Internet Explorer 10 in Windows 8 will be a plug-in free experience. It is available as a Metro style application and as a desktop one. The desktop app continues to fully support all plug-ins and extensions. In order to view Flash and other plug-in content in Windows 8, the user has to switch to the desktop application of IE 10 in order to see it.
Microsoft's decision came after examining 97,000 web sites, and discovering that 62% of them use Flash, but many of those need it solely to display advertisements and the majority of them fall back to HTML5 if Flash is not supported. YouTube is one example of this and many others have followed suit thanks in part to the huge popularity of portable computing devices like smartphones and tablets. Interestingly, Microsoft was mum about Silverlight support but we assume it will also be limited to the desktop version of IE.
For more information, please visit this blog entry from the Windows engineering team.