This is post is going to feature some spoilers for the season finale of Lost, so read on with that warning in mind...
The question about Lost has long been about how it can avoid making the mistakes of other long-running cult shows. A short while ago, they responded to the charge that, so long as it's profitable, the show might just drag on until it's tired and directionless (a la the X-Files) by announcing that there will be three more seasons at 16 episodes apiece. But with the finale of season three, the show has answered the biggest question of all: how do you maintain interest in the central mystery of the show (the island, what it is, and how they'll escape) without pissing off your viewers by being obviously avoidant (as with the X-Files, again); conversely, if you give your viewers the satisfaction of resolving the mystery, how do you maintain their interest? (this being the problem with Twin Peaks)
The answer, if you're Damon Lindelof and Carlton Cuse, is that you pretend to do both while actually accomplishing neither - the finale is all about misdirection and only appearing to give us what we want. They wrap up the plot of rescue but leave the matter of the island's nature itself dangling, a conciliatory gesture that is satisfying in that it resolves a plot point that was going to get tired very soon but maintain the most engaging mysteries. And, to top it off, they even introduce some more provocative questions. Really, it's brilliant stuff.
At the end of the episode, we learn that the survivors have been located and are soon to be rescued. We also learn, via the flashback that is revealed to be a flashforward about two and a half years into the narrative future (as confirmed in a screen-capture of Jack's newspaper), that this is no ruse - for better or worse, they will be rescued. The central conflict that has dogged our characters for 3 years has been resolved - they found a way off the island.
Or have they? Jack's flashforward* (*flashforward isn't technically correct - see my comment below) is a cautionary tale that the viewers would do well to consider: the dangers of getting what you ask for and all that. Jack is miserable and has come to realize that he never actually wanted to get off the island - and now he wants answers and intends to return. Cuse and Lindelof have found a narrative justification for returning to the island without being forced to concoct ridiculous or overly complicated reasons to keep the Losties trapped and unable to contact the outside world. Now they (well, Jack, at least - but it's reasonable to assume he isn't alone in this) want to return and we want to return with them.
This is also a very cute way to get around certain practical issues endemic to producing a show of this unique sort: moving the time-frame ahead by 3 years or so allows for them to update each character's appearance and admit that they've aged, which was soon going to demand too much of our sense of disbelief. It also allows for the cast to change - to bring, say, Walt back or introduce someone totally new without having to claim once again that they were always there or someone stumbled on to the island. These tricks have been done, and now Lindelof and Cuse don't need to rely on them anymore. (Assuming, of course, that season four will now relocate the narrative present in 2007 or 2008 to show the survivors returning. But why wouldn't they, given that the ambiguity of their fate was what held our interest in the narrative frame of December 2004?)
I'm not going to speculate on matters of plot or theme, here, though the shoe does much to prompt plenty of new questions of this sort: who was left behind on the island (the Others, I assume - Locke too?), what has happened to the island over the last 2 and a half years (have those who remained been killed?), who's visitation did Jack attend (Michael's is my guess), and how will they ever get back are only some of the more interesting questions. But in closing I'll return to the Biblical thread that I visited yesterday: both the biblical tale of Joseph's brothers and of Moses (with whom Ben compared Jack in the finale) feature prolonged periods of suffering and travel before an eventual return. In Joseph's story, the family is eventually reunited but only after his brothers have been tested; in Moses' story, the people eventually reach their promised land, but only after a war and the death of Moses himself.
Which is simply to restate the obvious: that the flashforward isn't an indication of how the story ends, nor is the rescue the beginning of the end. The end of this first part or series, maybe. But if you think they'd leave us with an alcoholic, pill-addicted protagonist without offering him the opportunity to redeem himself in some way, you haven't been watching this show for the last three years.