Dean Hachamovitch on the Building Windows 8 blog:
Running Metro style IE plug-in free improves battery life as well as security, reliability, and privacy for consumers. Plug-ins were important early on in the webâ€™s history. But the web has come a long way since then with HTML5. Providing compatibility with legacy plug-in technologies would detract from, rather than improve, the consumer experience of browsing in the Metro style UI.
Let me just add to other quotes to this:
Adobe has posted its reaction and says that it will offer Air-based Metro Apps fÃ¼r Windows 8.