DANGER!!! SPOILER ALERT!!! (For those highly sensitive to such...)The first section is all stuff that has been made public, ie, either released in official press announcements or said by the producers.
1)The first episode is called LA X (I believe it's two hours, so it's LA X Parts 1 and 2)
And yes, the space between the "LA" and the "X" is intentional. We pick up right where we left off last season with Juliet blowing up a bomb in 1977 Dharma times and Un-Locke egging bitter Ben to kill the mysterious Jacob.
2)Juliet (Elizabeth Mitchell)will be back for an unknown amount of episodes (or maybe just one)but won't be a regular.
3)Nestor Carbonell aka Richard aka Manliner Man is now a series regular. We will learn a LOT more about Richard.
4)Emilie de Ravin (Claire) is back as a regular.
5)Many dead,missing and long since seen characters from the past will reappear next season. Dominic Monaghan appeared but did not speak at the Lost forum at ComicCon this year. He'll be in at least 3 episodes. Ian Somerholder (Boone, the first big Lost death)will also be back for a couple of episodes. It was not released in what capacity they would appear...flashbacks, dream sequences, ghosts, or alternate timelines.
6)At Comic Con, videos were shown of Hurley in a Mr Cluck commercial apparently filmed AFTER the successful landing of Oceanic flight 815 in Los Angeles, and a segment of America's Most Wanted featuring the actual host John Walsh focusing on the search for the notorious Kate Austen, but not for the murder of her father, as we learned in Season 1, but that of another man who was mistakenly in the house that Kate blew up. Apparently, a different alternate timeline will be explored in Season 6...
7)Producers did confirm that we will see Daniel (Jeremy Davies) again but did not specify in what capacity (flashback, dream, alternate timeline). And, we will see Desmond in LA X.
WARNING...MORE SPOILERY STUFF AHEAD LEARNED FROM FANSITES AND NOT OFFICIALLY RELEASED. STOP READING NOW IF YOU ARE SENSITIVE TO SPOILAGE!!!
8)Maggie Grace (Shannon*, Boone's stepsister) is listed on the IMDB as being in LA X.
9)The following characters have been seen on set/location in Hawaii: Ilana, Bram, Charlie*, Boone*, Doc Arzt*, Kate's Marshall*, Frogurt*,Cindy the stewardess, Zak and Emma, Ethan*, Lapidus, Rose, Bernard, Charles Widmore. (*indicates dead characters). No word if anyone has sighted Ana* or Mr Eko*, but Adewale Akinnuoye-Agbaje said at Comic Con he was open to return to the show. Jorge Garcia (Hurley) said in an interview recently that it looked unlikely that Cynthia Watros (Libby*) would be returning, though her character will probably be mentioned. And Harold Perrineau (Michael*) said he is open to coming back as well, if the script is good.
10)A big Temple set has been constructed.
11)Jacob is the father of a character we already know.
12)Episode 3 is "What Kate Does" and is Kate-centric, (duh).
13)Episode 4 is "The Substitute" and is apparently Locke-centric. Locke's ladylove, played by Katey Sagal, will appear and his shitty little boss Randy Nations(who used to be Hurley's boss) played by Billy Ray Gallion.
14)Scenes are being shot that take place on the Island circa 2007 (last year, the time frame Un-Locke, Ben and Sun were in when they went to the base of the statue) with that cast but there also seems to be Island set, 2008 scenes with Jack, Kate, Hurley, Sawyer, etc meaning that they are no longer in 1978/Dharma times. And, most importantly, they are filming scenes of an alternate timeline featuring the characters AFTER Oceanic Flight 815 successfully lands in Los Angeles in 2004.
15)No word on the status of Jack's chest pelt...
