Windows 8 will have two versions of Internet Explorer 10: a conventional browser that lives on the legacy desktop, and a new Metro-style, touch-friendly browser that lives in the Metro world. The second of these, the Metro browser, will not support any plugins. Whether Flash, Silverlight, or some custom business app, sites that need plugins will only be accessible in the non-touch, desktop-based browser.
Should one ever come across a page that needs a plugin, the Metro browser has a button to go to that page within the desktop browser. This yanks you out of the Metro experience and places you on the traditional desktop.
The rationale is a familiar one: plugin-based content shortens battery life, and comes with security, reliability, and privacy problems. Sites that currently depend on the capabilities provided by Flash or Silverlight should switch to HTML5.