The bad news is that the adventure update has been delayed, but the good news is that version 1.7 will come out on time. Confused? Explanations are inside.
According to The Word of Notch the adventure update "had been growing in scope" and will take "some time to finish" but instead of taking too long between updates he decided to split the update into two. Thus patch 1.8 will now be the adventure update and patch 1.7 will be a smaller update without the 'adventure' aspects.
Here's the information (and speculation) we have on the two updates: