While Minecraft's big, shiny new Adventure Update was intended to be released in the near future, it seems it's already been leaked, and the folks at Mojang have decided not to sweat it/likely leaked it themselves. From Minecraft creator Markus Persson's twitter feed:
Ooh, looks like 1.8 has leaked, not that I'd ever admit that. ;) Be careful when installing software you don't trust though.
Further, Mojang has (more or less) officially offered a 1.8 pre-release through Minecraft Forum, which offers even more information than has been previously available about the Adventure update.
Having tried out Minecraft Beta 1.8 this morning, I can attest that the alterations to the game are both enormous and incredible. The lighting overhaul is spectacular, with different colours for different light sources, and a much smoother look overall; pine trees have been overhauled, with many growing to great height now; the number of animals spawning seems to have been upped, probably to allow for more readily-available food; abandoned NPC villages are God-sent for those lonely first nights; biomes range from their current size to absolutely enormous - forests and oceans stretch across the horizon - and you really, really have to watch out for abandoned mine shafts and canyons. I can't emphasize that last point enough. At first blush, the Adventure Update really does help fulfill the developers' wish to make Minecraft feel more game-like and less like a Beta. There's a lot more to 1.8 that I really don't have space to talk about...and is probably best experienced first-hand anyway. That being said, the Adventure Update does feel a bit like a pre-release - which, of course, it is. I found it much slower and much more of a RAM-hog than previous versions. My laptop, which is a few years old, struggled to keep the frame rate up to a reasonable level even after I toggled some of the handy new video settings for optimum performance; meanwhile, the game fared much better on my husband's desktop (which is also a few years old) though it experienced a few more temporary chunk errors than in previous versions.
Regardless, after a few hours' play 1.8 feels like an enormous step up, and leaves me excited for the chance to try it out in survival multiplayer. Remember, folks, this is a pre-release, meaning bugs are to be expected and there's still tweaking to be done on the development side. But if you want a taste of what the official release of the Adventure Update is going to be, head on over to the Minecraft Forums and download it. (Installing the pre-release is dead easy for us Linux users: Just replace the old .jar in /.minecraft/bin/ with the new one - and do save a copy of the old one just in case. It's a tad less intuitive for you Windows people, but there's a tutorial already up on the forum for you.)