Adventure Mode, formerly Dungeons and Levers, is an upcoming game mode.
Currently, focus on adventure is an update to the base game, not a separate mode. This "Adventure Update" should not be confused with the possible Adventure Mode itself.
Notch has said that the Adventure update (or similar) will come in Beta 1.8. , but Jeb confirmed that didn't mean Adventure Mode. It is more of a way to "flesh out the game a bit, making it reward exploration and combat more".
The Minecraft "Adventure Update" will be a huge update to Minecraft, similar to the Halloween Update. The Adventure Update was originally going to be Beta 1.7, but was later postponed to 1.8. It might be split into two updates due to its size and to its still unfinished status. The tentative release date for the update (at least the first half) is September 8th, 2011.
Notch replied on a tweet from @ZeroDegre that he is nervous about this update.
An early version of the Adventure Update was playable by fans at Mojangs booth at PAX Prime, which was held between August 26th and 28th. A user from the Minecraft forums who was picked as a volunteer at their booth provided inside information on some of the new features. More on this can be found in this thread. Mojang and Notch seem to be okay with this, as Jeb tweeted 'Yeah, we know' to someone noting that any new details would leak from PAX in a matter of hours. Note that despite letting fans get a sneak peak at the update this does not necessarily mean that it will be released in the days after PAX. Jeb twittered that he doesn't know exactly how long they will continue to work on it and that he can't say that they're close to the release date. This could be due to them not knowing which and how many bugs will turn up during PAX, and subsequently not knowing how long it will take to fix them.
For a list of features in the Adventure Update, consult the Upcoming Features page.
Notch has also stated that the 1.8 update will come out after the 27th of August After PAX).
On September 3, Jeb has stated on Reddit, that Beta 1.8 (Adventure Update Part 1) may be released on September 8. (Jeb's Quote : Best estimate 8th of September)
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