The leak has been made available first on private ftp servers and other private sites, before it made its way to the rest of
the internet.You can read more about the leak here on Windows 8 News. The screenshots are the first as far as I know that have been posted publicly.
Windows 8 News has all the Windows 8 screenshots
for you. The good news is, you not only find the leaked screenshots of
the Windows 8 version on the site, but also annotations that describe
new features that are shown on each individual screenshot.
The leak was installed on a tablet PC and the user who submitted the photos managed to get the metro UI working on the tablet. The metro UI was not the one built into Windows 8, but a Rainmeter theme.
I'll post more soon about this