In the Nikos Kazantzakis book (and possibly in the Martin Scorcese movie - I've never seen the film version), the titular Last Temptation is offered to Jesus by the Devil while he was in the act of dying on the cross.
In this Temptation, the devil offers Christ a vision of another reality, a reality in which Christ foregoes the like of a martyr, and instead lives a quiet, peaceful, "normal" life in his home village. He takes a wife, he raises children, he has friends and a community, and lives on for decades into his old age. After many years, he comes to understand that this is all a fantasy: a vision of a life he had to give up in order to fulfill his fate. The Devil tells him that he doesn't have to be a martyr, that he can abandon his destiny, and stay in this "reality" forever.
In the end, however, Christ chooses to die as he was destined to. He returns to the cross at the exact instant at which he left it, and the rest is, as they say, Gospel.
This ties in too perfectly to Desmond’s experiences in Flashes Before Your Eyes. In FBYE, after the hatch implosion, Desmond is offered a vision of another reality. In this reality, he can give up his destiny as a Lostie to instead have his quiet future with Penny. In this instance, of course, he is forced back into the Island reality (as opposed to choosing it).
But my theory is that this Flash, this Instantaneous but Indefinite vision, is what the Alt Timeliners are going through. All of our main characters, thanks to the Incident, are being given a chance to see what their worlds could be like if they simply rejected the Island. Jack gets a son. Claire gets Aaron. Locke gets Helen. Sawyer gets a life free of crime (and, I think we all predict, will get a second chance with Juliet before all is said and done).
Smokey is tempting the candidates/Losties. He’s giving people what they want if they only reject Jacob’s calling. “Look, see Sayid, your beloved lives, and will continue to live, if you just choose not to come back.”
Before this season ends, the Alternate Losties will be given a choice, maybe by Eloise Hawking, to accept their destiny and sacrifice for the world’s greater good (keep Smokey on the Island, preventing him from wreaking havoc on the world), or to reject the needs of the world for the chance at their own personal, private fulfillment.
That or Eloise did what she did to Desmond, says “nope, sorry, not for you,” followed by a *WHACK* that sends them back to the Island, where they must either accept their fate there, or be tempted by Smokey when he promises them that world again.
If, as we learned in Ab Aeterno, all their souls are up for grabs in a contest between Jacob and Smokey, where Smokey will try to tempt and corrupt them all to defeat Jacob, it makes sense that the Alternate Reality is, in fact, their Last Temptation.