The recent leaks of Windows 8 suggests that there are sources within Microsoft that are letting out copies of the work-in-progress operating system. According to a well known leak tracker, as characterized by WinRumors, Microsoft has fired two employees that the company believed to be leaking the beta builds.
MyDigitalLife forum member Canouna posted that two employees were fired, and that Microsoft was able to trace the leak of at least one of the Windows 8 beta builds back to its origin.
The termination of employees cannot be verified, as Microsoft does not release such information. Rumors claim that Microsoft fired one of its employees in Redmond and another in Russia.
Given the words of warning that are clearly displayed on the desktop wallpaper of the Windows 8 betas, this shouldn't be surprising action by Microsoft.
I don't think Microsoft would take kindly to their employees putting current or past versions of Windows out there for download, either.