When the announcement of the preview was made the bits were not yet available for download; They are now. Head over to http://windows.microsoft.com/en-us/windows-8/preview-iso and grab yourself a copy.
Product Key: NTTX3-RV7VB-T7X7F-WQYYY-9Y92F
Important: Windows 8.1 Preview isn’t currently supported on some tablets and PCs with newer 32-bit Atom processors. Get the details here