The leak has been posted on the BetaArchive forums and has been confirmed by several reputable sources that it is indeed a legitimate build, according to Neowin. It's a matter of time until this gets posted to a public torrent tracking website and then spread very quickly, meaning it will be available from your favorite website for leaks.
The build that has leaked has the following file name: "6.1.7850.0.winmain_win8m1.100922-1508_x86fre_client-enterprise_en-us.iso." In other words, this is quite an old build, given that it is from Milestone 1, screenshots for which leaked three months ago. In fact, the build string shows that 7850 was compiled on September 22, 2010.
As such, if you grab this build, you won't see many of the leaked and rumored features we've seen so far. That means no native SmartScreen file checking, no built-in PDF reader, no Office ribbon in Windows Explorer, no new logon screen, no system reset feature, and no other features we've discussed but don't have screenshots for yet. In other words, it will look very close to Windows 7.