For more details visit http://blogs.msdn.com/b/b8/archive/2011/09/13/welcome-to-windows-8-the-developer-preview.aspx
IMPORTANT: Upgrade from Windows 7 installation is not supported for pre-release code; only clean installs are supported. Reminder: this is a developer preview release and is not meant for production. It is not a beta release. We will be updating the release with various quality updates and drivers over the coming weeks/months just to exercise our overall update and telemetry mechanisms.
And if you plan on developing any applications, be sure to download the 64-bit version, as the tools are not included on the x86 build.
Downloading as I type. Will this install as a VM in Hyper-V?