Notch, the creator of Minecraft has just confirmed the legitimacy of a leaked pre-release of Minecraft 1.8. The files leaked have been confirmed real and are ready for downloading from the pre-release Minecraft servers now.
Notch has posted on twitter confirming the leak of Minecraft 1.8, he says "Ooh, looks like 1.8 has leaked, not that I'd ever admit that. 😉 Be careful when installing software you don't trust though."
He also warns to make sure you download the pre-release from Minecraft.net, not third party download sites. He also wishes for those who are using the new 1.8 pre-release to report bugs.
Opeth, one of the mods at minecraftforum.net says "As with major version releases, prerelease copies are normally distributed to trusted users and developers for bugfix testing and to enable mod updating in time for the official release. A version of the prerelease has been publicly leaked by one of the groups."
You can read the official post from Opeth here. Download links are available at the given link.