UPDATE: 4th October - After reading the reports of the Premiere a lot of this can be confirmed as a hoax.
From the Tailsection we find the following."Hey Doc! Call me 'Super-Sock'. We had a small party and under threats of death if any details are leaked, we were allowed to watch a "Work Print" of "Tale of Two Cities". First the overall impression. I know alot of people are saying that they will drag out the reveal of who the others are and what they are doing, I was stunned by how much was revealed. It wasn't 100% but you will know, roughly, what they do and how long they have been at the 'island'. I quote that because, honestly, I'm not so sure it is an island. There was a lot of unfinished effects work in the print we watched but it was clear they wanted to give it the look of an artificial land mass. But on a scale that no human could ever build. The episode is mind game after mind game. First you think their free, then their captive, then their free again. The "others" idea of testing is seeing how they will react in different situations. In the end of it, Fenry makes Jack an offer which will just blow your mind, but it looks like another test. It ends with Hurley finding Desmond naked in the Jungle. There are some very cool things shown, there is a connection between Sarah and Locke, there are definite signs in the flashback that there were forces at work pushing Jack, at least, towards his encounter with a predetermined destiny. Jack is definitely left in a very strange headspace. There was no music and the effects were only partially done but it seemed like there was a LOT of CGI. Oh, and at one point you definitely hear the monster in the others habitat. There is no doubt he is "one of them". '
Source: The Tail Section