The activation has been performed by using the KMS server activation method. The KMS server activation method has been shared by the same person who leaked the Windows 8 RTM ISOs. Now, the users are able to customize their Windows 8 RTM SKU like they want but, the activation would remain for 180 days and after that you will have to re-active.
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For those of you, who downloaded the RTM SKU, can search on internet for the activation methods. But, for those who don’t want to use is illegally can wait for the official launch of Windows 8 on October 26 when it would reach general availability milestone.