Notch has announced that the 1.7 update will contain the usual bug fixes and pistons, the adventure update will be arriving in the 1.8 update.
That means 1.7 will be released soon, containing only pistons and probably a couple of bug fixes. The adventure update will take however long it takes, and will be released as 1.8. It might be a long wait, but it will be worth it!
See The World of Notch blog for further info.