Earlier this year some scamp edited together 20+ minutes of trailers, TV spots and internet clips of The Amazing Spider-Man into a fairly correct chronological order, creating a shortened version of the movie. It turned out Sony had revealed every single last beat of the film in their advertising.
That was the extreme case. This video does the same thing - less skillfully, I'd say - for The Hobbit: An Unexpected Journey, and only runs about seven minutes. I'm actually unsure if the order presented here is even correct; there's a lot of non-book material that the editor has put in, but it's unclear if he put it in at the right points. One thing I think this assembly does reveal is that a lot of the film will take place in Hobbiton; with the series extended to three films I bet we spend the first half hour or more at Bag End.
We'll know soon enough. The first press screenings of The Hobbit are happening within days.