Early adopters putting the first public pre-beta of Windows 8 through its paces know by now that not only is a product key not provided by Microsoft along with the ISO images of the Developer Preview Build, but that it’s also not needed during the installation process.
There’s a simple enough reason for this, Windows Developer Preview comes with the following product key out of the box: 6RH4V-HNTWC-JQKG8-RFR3R-36498.
This being said, feedback indicates that testing Windows 8 is not always a perfectly smooth experience, but Build 8102 Milestone 3 (M3) is a pre-Beta release after all.
Installing a Product Key and Activating Windows 8 Developer Preview - Softpedia