Microsoft has just posted that they will be releasing the developer preview of Windows 8 today for download:
Starting at 8PM today, Seattle time, you can download all of the code that attendees at BUILD received. This includes 32 or 64 bit x86 builds, with or without development tools. The releases also include a suite of sample/SDK applications and the SDK (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. You download with a Windows Live ID (which you might want to use to test out some of the new roaming features). 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.
The download can be found at dev.windows.com
Read more at Microsoft here.