During a panel about the new season, they talked a little about flashforwards:
http://www.comicbookresources.com/news/newsitem.cgi?id=11461“One of the things that we wanted to do since the very end of season one... is the flash forward,” Lindelof confirmed. The third season finale ended with the revelation that Jack and Kate had somehow gotten off the island and that we had been seeing “flash-forwards” throughout the episode, not flashbacks. The panel also announced that in the new season, the show will feature both flashbacks and flash-forwards. “The operative question is how forward and with whom. But if you continue to watch the show you will be pleasantly surprised,” Lindelof said.
The creators felt they had to wait to introduce the flash-forward until they knew they would be allowed to end the series, and when. It was announced in May that ABC would air three more seasons of 16 episodes each, beginning in February 2008.
“Previously, the storytelling of the show had taken place in the present and had gone into the past,” Cuse explained. “We always wanted to introduce this notion of the future. Now with an end date we have the certainty to allow ourselves to do that. Once we knew, OK we're down to 48 episodes, we thought, now we can put it out there.”
Lindelof also clarified the end of the Season 3 finale. “We want to communicate that what you saw with Kate and Jack was not the end of the show, as has been suggested. The show has never really been about getting off the island. There's this whole chapter of our story that takes place off the island but we couldn't start telling it until we told the audience that the show is going to end. If you had seen the Kate and Jack scene and it was just another episode, as opposed to one less episode, I think you would have thrown up your hands and said, ‘They are fucking stalling again!' But because we're really moving towards the end now, we felt we owed it to the audience to do something radical and say to everybody, ‘Come with us all the way to the end, it won't be a waste of time.' And ABC let us do it, which is cool.”
Check the article for spoilers on who's coming back.