Once again, let me reiterate, there are MAJOR spoilers ahead. Do not read if you don't want a LARGE part of the game spoiled.
AGAIN. I MEAN IT.
YOU'VE BEEN WARNED.
OK, here you go:
This doesn't have the official stamp of approval, but a hack of d3.exe has surfaced this morning containing a LOT of information on D3. I've spent the day combing through it, and I'll paint out the story in broad strokes:
You begin by rescuing Cain and killing Skeleton King in order to reach the meteor. Once there, you find out that the meteor is actually a man, referred to in the leaks so far as a "Stranger."
After here, the information becomes a bit more speculative, but I can tell you that Act I concludes in a place called the "Halls of Agony," ushering you into Caldeum, and Act II.
Act II seems to center around a boss character named "Saddle," by whom I assume they mean Belial. Act II ends in a place called the "Archives of Zoltan Kulle."
As well, there are numerous references to a character known as "Jumper," who appears to lead the cultists, near as I can tell. Act III, as many have already surmised, centers around Bastion's Keep and (obviously) concludes within the Arreat Crater.
For those who suspected that Act IV would take you into Heaven, it appears that you may be right. With location names such as the "Silver Spire," and "Gardens of Hope," there appears to be a blurring of the boundaries between Heaven and Hell. This is somewhat speculative, but from what I can surmise from what I've seen, the Great Conflict appears to collide with the mortal world at the conclusion of the game, which, incidentally, is also Bastion's Keep.
There's a bit more information, such as samples of dialogue and such, which you can review to your liking here:
Many of you have guessed that the 3 Primes will merge into one being. While I can neither confirm nor deny that this part is true, there is a bit of the "prophecy" which seems to lean in that direction as well, so you kuhrazy kids may be onto something.
Generally, monster prefixes and suffixes now tend to relate to their enchantments. Joy.
It's all there, item prefixes, suffixes, locations, monsters, even some bonus stuff.
Apparently somewhere in there, there's a "battle for ponies."
....Your guess is as good as mine. In any event, I'll let some other folks look over this now. I'mma go have a sandwich. Have a good day, all!