We all know how our beloved couple Darlton said, very early on the series, that there would be no time travel on the show. Many of us felt tricked because of Season 5 and its obviously time travel-based plot.
I don't think our legs have been pulled here though. My opinion is that they said what they said because the ultimate answer that we've been wanting for five years now, the answer to "Guys, where are we?" has nothing to do with time travel and that the latter was just a plot device that Darlton discovered along the way, much in the same fashion they "discovered" the Flashforward.
If you listen to their Podcast (if you don't....what are you doing reading theories here?), you'll remember that they realized that the Flashback device wasn't going to work if used six years in the same way, every single time. So they thought: which storytelling device can we use to tell this story in a better and more interesting way? The Flashforward.
So this is probably what happened when they planned the rest of the show, in the middle of Season 3 (remember that the time travel device was, inf act, introduced in the Season 4 finale, with the Island moving both in time and space). They thought that time travel was the better way to tell all the Dharma, Destiny and Predestination storytelling that they wanted to tell.
The reason I say all this is to show you why I think the ultimate LOST answer has nothing to do with Time Travel - and, maybe risking a little bit, I dare to say that even the whole Adam and Eve issue has nothing to do with time travel. I know it seems not only the most logical answer but the ONLY possible one, but remmeber that it was there since Season 1 and it supposedly will be the confirmation that they KNEW the ultimate answer all along.
In a nutshell, they clearly weren't thinking about time travel since the beginning, but they knew the overall plan since the pilot. Ergo (haha!), Time Travel is just a big red Herring to us - well, not only this, but it is NOT the definitive answer to LOST.