Minecraft 1.8 or the Adventure update has been leaked as announced on Notch's Twitter feed.

Judging from the Twitter posts, it seems quite obvious this isn't really a leak but an intentional unofficial release to allow some public testing before the actual update is released and some users are reporting bugs and issues and Notch has said that users can use the unofficial release and encourages them to report bugs.

That and the fact that the leaked update is available at the below link that resolves to the minecraft servers, it should kind of remove any doubts that this is anything other than an unofficial release from the company to get some public testing done prior to release.

http://t.co/GIFIluz

(Though I have to say, it wouldn't even load for me for some reason.  The Jar file refused to load with a main class not found in jar file error.)