I cannot be held responsible for any of this content. It may possibly be accurate or it all might be a load of shit. Either way, it's damn entertaining.
Courtesy of WatchLostOnline.net:
Despite the fact that many of us (myself included) suspected that Christian Shephard might actually be Jacob, we finally have confirmation that he is not. The role of Jacob has been cast and he will be appearing in the Season 5 finale of Lost. Mark Pellegrino will be playing the mysterious island leader. And he won't just be appearing....he will be interacting in flashbacks with some of our main characters.
Jacob: We will learn that Jacob has witnessed pivotal events in the pre-island lives of Locke, Sayid and Sawyer. Source: EW
Sawyer: Jacob will meet a young Sawyer at his parents' funeral. Jacob will not be interacting with young Sawyer for very long, but he will be acting as a sort of conscience for Sawyer, telling him something like "I know what you did". Source: The Observ3r@The ODI & The Transmission
Locke: We will see a scene where Mark Pellegrino (who has been cast as Jacob) is reading a book on a bench outside a building when BAM!! Locke falls eight stories out of the window and lands behind him. While people cry out for help and to call 911, Pellegrino (Young Jacob?!) calmly puts a bookmark in his book and casually walks over to Locke's body in the grass and kneels down next to him. Source: Dark UFO
We will definitely be seeing Jacob's cabin again. Source: The Observ3r@The ODI
Here are some non-Jacob related spoilers:
Juliet: The sub will make a reappearance and Juliet will board it again (in a flashback maybe?). Source: vaudesir@Flickr
Sayid: The finale will feature the scene where Sayid's wife, Nadia, is killed. Sayid will either witness her death or see her dead on the ground. Word is that Sayid will also be injured (probably doesn't relate to Nadia's death since we saw no earlier indication of this). Source: The ODI
Sun & Jin: Their weding will be featured in a flashback. Source: The Transmission
Losties: We will see a return to the beach camp (and we will see Rose and Bernard again!). Source: The Transmission
Penny: While we know there will be a death of a (semi-major) character by the end of the season, it will NOT be Penny. Source: E!Oniine
Rose & Bernard: We will find out what happened to Rose and Bernard during and after the time-travel. Source: EW
The Temple: We will be seeing inside the mysterious temple in the season finale. To get inside, people will have to get a bit wet. It's very Egpytian with large columns around a type of fire pit in the middle and torches on the wall. There's a tapestry on the wall with Egyptian figures on it. At the very top of it, some figures are kneeling to a sun with an eye in it and the sun's rays have hands on them. Next to the tapestry is a spinning wheel. Locke and Ben will be inside the temple. Source: and DarkUFO