Starting with a rollout tomorrow, the Metro-UI (Modern UI) version of Internet Explorer 10 in Windows 8 and Windows RT will automatically allow websites to run Flash unless they’re on a block list. Previously, this version of IE10 had a whitelist where Microsoft could store safe sites for Flash, or you could view the sites in the Desktop Mode. There was also a hacked editor where you could edit the whitelist.
Looking at our engineering experience with Flash and Windows 8 and RT, as developers improve their Flash content, the vast majority of sites with Flash content that we have tested are now compatible with the Windows experience goals. Of the thousands of domains tested for Flash compatibility to date, we have found fewer than 4% are still incompatible, in the most part because the core site experience requires other ActiveX controls in addition to Flash. With Windows 8 in the hands of customers and developers, we listened to feedback around the experience of Web sites with Flash.
Source: MSDN Blog