There's an excellent shot-by-shot analysis of the prelude to Melancholia by Manohla Dargis in the New York Times.
Road to the wedding
One of the things Dargis suggests is that some images and references in the prelude hint that Part II of the movie might all be in Justine's head, like Oz or something. Usually I think that this kind of thing is pointless, because the way movies get in our head is more important than figuring out some cute puzzle about which character's head the action is taking place in, but Dargis is pretty convincing.
Yellow Brick Rd
The idea that the apocalypse is imagined by Justine gives some explanation to one visual detail in the movie. In shot #5 in the prelude (and I think we see it again in the second half of the movie), the number 19 is on the flag on the golf green, one more hole than golf courses have. And von Trier even makes a point of having Claire's husband mention later that they have 18 holes on their course. I was thinking that this was a little joke about the world of this movie taking place after the end, on the 19th hole, but maybe that's a bit literal.
It's curious too that there are suggestions that Part II is a projection of Justine's consciousness, but it's the first part that's named after Justine, while the second is named for Claire.
This whole idea reminds me of when I saw Meshes of the Afternoon in college and attacked it in class for copping out by having the whole thing be a dream at the end.
On the road again