According to a post made by Steven Sinofsky on the official “Building Windows 8″ blog, which provides an inside look at the upcoming operating system, Microsoft will be releasing a developer preview of Windows 8 “later tonight.” The operating system preview will be available in both 32-bit (x86) and 64-bit (x64) builds, bundled with development tools and a suite of demo applications. Upgrading from Windows 7 directly is not supported, so you’ll have to do a clean install to run this pre-release version of Windows 8. Sinofsky stresses that this is only a developer preview, rather than a beta release, and isn’t intended for production.
You probably want to try out the preview release—and you can. Starting later tonight you can download the Windows 8 Developer Preview. This includes a 64-bit (x64) build with development tools to build apps, and a 32-bit (x86) or 64-bit (x64) build without development tools. The releases also include a suite of sample applications (please note these are merely illustrations of potential apps, not apps that we intend to ship with Windows 8). The ISOs are linked to from http://dev.windows.com.
UPDATE: It’s live — http://msdn.microsoft.com/en-us/windows/home/