I know we've said this 100 times, but next week seems like an episode that has to be packed with new info. Jack has spent a lot of time with the Others. He has talked to a lot of people, including some of the tail section captives, perhaps. He has all sorts of personal knowledge of how they were living. He has to have learned something interesting related to how they go back and forth between the two islands. When Jack talked to Kate 2 weeks ago right before leaving, the viewer was forced to believe that he couldnt talk to her in detail because they were listening in. Now, he should be ready to talk.
Juliet may or may not be forthcoming and may or may not be honest if she is, but if the writers are fair, we should learn a great deal from Jack.