Windows 8 Release Preview Setup will check to see if your PC can run Windows 8 Release Preview and select the right download. Setup also features a compatibility report and upgrade assistance. Make sure your devices are connected to your PC and turned on before you run Setup. Built-in tools for creating an ISO or bootable flash drive are available for some previous versions of Windows (excluding Windows XP and earlier). You can find system requirements and additional information in the FAQ and in the links on this page.
Note before you download: Windows 8 Release Preview is prerelease software that may be substantially modified before it’s commercially released. Microsoft makes no warranties, express or implied, with respect to the information provided here. Some product features and functionality may require additional hardware or software.
If you would prefer not to use Windows 8 Release Preview Setup, you can download Windows 8 Release Preview in ISO format instead.
64-bit (x64) Download (3.3 GB) Sha 1 hash — 0xD76AD96773615E8C504F63564AF749469CFCCD57
32-bit (x86) Download (2.5 GB) Sha 1 hash — 0x8BED436F0959E7120A44BF7C29FF0AA962BDEFC9
I strongly suggest that anyone that plans on downloading and testing the release preview backs up their system before hand and/or does not test it on a crucial system. There have been no reports of issues but this is still software that is in the testing phase and it might not work with all your applications and recovering from the update might be a tremendously tedious process.
For more details about the release preview checkout this post on the Windows 8 blog "Delivering the Windows 8 Release Preview"