Thanks to a flaw in Microsoft's various activation methods for Windows 8, pirates are able to receive a free, legitimately activated copy of Windows 8. Through a combination of an illegitimate KMS activation followed by a free Windows Media Center upgrade, anyone is able to receive a fully activated "legitimate" copy of Windows 8 with very little effort.
The basic method involves first installing Windows 8 with any key (even one already in use), and then following up on this by using a KMS (key management system) server that accepts any key as valid. This will give you an illegitimate installation of Windows 8 for 180 days, as KMS activations need to report back to the server every 180 days.
To turn this into a legitimate copy of Windows you simply need to apply the free upgrade for Windows 8 Pro that adds Windows Media Center, which during the installation apparently doesn't check if the current key in use is valid. Each Windows Media Center upgrade key is unique to the system it's installed on, and replaces the previous key for a fully and permanently activated copy.
The free upgrade to Windows Media Center is a limited time offer that only lasts until January 31, 2013, so there is only a few months for pirates to use the method assuming Microsoft doesn't close the holes before then. Plus, with the relatively cheap cost of upgrading to Windows 8, there's little reason to use a potentially dodgy activation method like this.