The source is Andrew Lawden, a cosplayer who has been hired to appear at various promotional appearances and the like as Iron Man on behalf of Marvel. He spoke with Bombad about some of what he's privvy to by v irtue of spending so much time around the Marvels, and it's pretty impressive stuff.
Says Lawden (transcript by Screen Rant):
“Thor becomes reunited with Jane who becomes possessed by a Dark Spirit, which is connected with the Dark Elves; a race the Asgardians have fallen out with many years beforehand [...] The Dark Elves attack Asgard, and they take a huge hit. So Thor has to rebuild Asgard by putting together an army to go and take the battle to the Dark Elves and defeat them. But the only way he can do this is – he needs to cross over to the Dark World, and the only person who has access to the dark side of Asgard is his old brother Loki, who at the beginning of the film is locked up in prison.Of course, the rumor that "someone dies" has been circulating for a while now, but up until this point, it wasn't Frigga but Odin himself who was rumored. That change is not an insignificant one, given that Odin plays a much larger role in the comics than does Frigga--and did so in the first film, as well.
“So what happens is Thor basically has to negotiate this truce with Loki, and he goes on this great trek with Jane, all three of them. They go to this far side of Asgard, which is being shot in Iceland, where they train and assemble an army to go and attack the Dark Elves’ world.
"One of the things that happens during the Asgardian attack is that Thor’s mother gets killed so it becomes like a vengeance thing. And they were keeping very tight-lipped about what actually happens with Loki. You don’t know whether he redeems himself or he’s still the bad egg."
That summary also explains why Marvel would be hesitant to let Natalie Portman out of her contract, if indeed rumors that she wanted to leave the franchise are true. It's possible they cast her specifically with a larger arc of becoming a villain in mind--and even if they didn't, there's a good chance that they thought they couldn't get anyone else of comparable "market value" to play the expanded role better in the time they had before they wanted to start principal photography.
Thor: The Dark World will come to U.S. theaters on November 8, 2013.