The Windows 8 Developer Preview which was released a few days ago is much easier to install that it's predecessor. So much so that you wouldn't have realized that it needs a product key just like the previous version. Well unless you have tried to re install it.When Windows 8 is installed for the first time it works like a charm. But if you have ever tried to re install the OS or use the Windows reset feature you would have been surprised with the requirement of a product key.
Microsoft didn't provide a product key officially but did post some keys in response to a question raised by a forum member.
"If you need to reinstall Windows Developer Preview or use the Reset functionality, you might be asked to enter this product key:
6RH4V-HNTWC-JQKG8-RFR3R-36498
If you’re running a server version of Windows Developer Preview, you can use this product key:
4Y8N3-H7MMW-C76VJ-YD3XV-MBDKV
Note: These product keys are only for use with the Windows Developer Preview version of Windows 8."
So if you have installed either the client or server version of Windows 8 Developer Preview and have the requirement for a product key you can use the keys provided above.