Lost: How Will This Season End?

There are two overall theories of time travel.  One states that time is a fixed constant and that, in the words of Daniel Faraday, "whatever happened, happened."  The other is that the past can be changed, creating alternate futures.  For an example of this, see Back to the Future and the disappearing photograph. The question on Lost is: which time travel theory does the show adopt?  Initially Faraday seemed convinced that time was fixed and that the survivors who traveled back to 1977 couldn't change the future even if they wanted to.  However, he almost immediately put a hole in that theory by talking to Desmond, who is somehow immune from the fixed constant theory. (link)