Players will be playing with pistons soon, but unfortunately, it will be a while before they're exploring underground ravines.
Minecraft creator Magnus "Notch" Persson says that the upcoming 1.7 update, which was supposed to be the "Adventure Update," which was supposed to add underground ravines, flying around on clouds, and a host of other features, will not be coming out for while, or at least, not in the form that players were expecting.
Writing on his blog, Notch said that the scope of the Adventure Update was getting bigger and bigger and that he didn't think that it would be finished anytime soon. However, rather than keep players waiting for an update, the adventure content is being hived off into a future update, and 1.7 will contain pistons as well as a number of bug fixes. Notch didn't give a timetable for when the 1.8 update would come out, just saying it that it would be finished when it was finished.
Post-1.8, Notch said the focus would be on getting the game ready for its "proper" release, which he said would be around the week after November 11th. He said that he and his team at Mojang would be working on improving performance, integrating single player and multiplayer, getting the mod API right, and generally added extra polish to the game.
Source: The Word of Notch