To install Windows 8 Developer Preview you do not need a product key for activation. However, you would be asked to enter a product key while re-installing Windows 8 Developer Preview Key.
If you need to reinstall Windows Developer Preview or use the Reset functionality, you might be asked to enter this product key:
If you’re running a server version of Windows Developer Preview, you can use this product key:
The key is provided by Microsoft employee BillFill in msdn forum. You can find the BillFill’s post here
Update 3/1: For Windows 8 Consumer Preview, you can use this product key:
Update 6/1: For Windows 8 Release Preview, you’ll need to enter this product key: