As the new season officially start on 4th Oct, I thought we could kick the spoilers off here

Wednesday, October 4: 9:00-10:01
Episode 3.1: "A Tale of Two Cities"
Jack, Kate and Sawyer begin to discover what they are up against as prisoners of "The Others." --Jack-Centric Season Premier-- Guest starring: John Terry as Christian Shephard, Julie Bowen as Sarah, M.C. Gainey as Mr. Friendly, William Mapother as Ethan, Brett Cullen as Goodwin, Blake Bashoff as Karl, Julie Adams as Amelia, Stephen Semel as Adam, Isabelle Cherwin as little girl, Alexandra Morgan as moderator, Sonya Seng as receptionist, Sally Davis as teacher and Julie Ow as nurse. Teleplay by J.J. Abrams and Damon Lindelof. Story by Damon Lindelof. Directed by Jack Bender.
CONFIRMED -- POST PRODUCTION

Wednesday, October 11: 9:00-10:00
Episode 3.2: "Further Instructions"
The fates of Locke, Eko and Desmond are revealed after the implosion of the hatch, while Hurley returns to the beach camp to tell the tale of what happened when he, Jack, Kate and Sawyer encountered "The Others." --Locke-Centric-- Guest starring: (among others) Ian Somerhalder as Boone Carlyle, Justin Chatwin, Christian Mulkey. Written by Carlton Cuse and Elizabeth Sarnoff.
TENTATIVE -- POST PRODUCTION

Wednesday, October 18: 9:00-10:00
Episode 3.3: "The Glass Ballerina"
--Sun-Centric-- Written by Jeff Pinkner and Drew Goddard. Directed by Paul Edwards.
TENTATIVE -- PRODUCTION

Wednesday, October 25: 9:00-10:00
Episode 3.4: "Every Man for Himself"
--Sawyer-Centric-- Written by Edward Kitsis and Adam Horowitz. Directed by Stephen Williams.
TENTATIVE -- PRODUCTION

Wednesday, November 1: 9:00-10:00
Episode 3.5: "The Cost of Living"
--Eko-Centric-- Written by Alison Schapker and Monica Owusu-Breen. Directed by Jack Bender.
TENTATIVE -- PRODUCTION

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New Characters
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Spoilers
WARNING: Some info may be foilers from TPTB.

Rumored to be in the Jack flashback, Sarah is talking to a male teacher who may be more than a friend, and a little girl ends the scene by telling Jack not to trust Sarah. A scene was filming at the Hawaii Convention Center, a Locke flashback at the Sydney International Airport. Locke was in his chair, but he was shirtless. He was being pushed by Boone, and he seemed to be pointing or gesturing as he's pushed through the crowd of travelers. It almost seemed like it was a "dream sequence." --The Transmission

The planned series finale of Lost [is] a "happy but tragic" ending. Kristen: "A little birdie told me William Mapother was in Hawaii last week -- and he exactly wasn't sipping mai tais by the pool, but rather, he was working his wonderfully creepy Ethan magic." Bryan Burk: "The Hatch is gone. Done." It looks as if Kate is spending some quality time in close quarters with good ol' Henry. --Kristen at E! Online

It will be revealed that Jack has met one of "The Others" back in the real world; this will relate to why he is the only one they are able to cooperate with, although he will be extremely wary in the beginning. --The Tail Section