Fantastic movie, and may be my favorite of this year.
The whole thing is a giant metaphor. Japan did their own thing, and there were references to them being the major players, as in the horror genre doing well there. It's America that has the cliche' characters:
whore, jock, nerd, clown, and virgin
There is some debate as to who the gods are, but IMO it's pretty clear that we (audiences) are teh gods. And the whole message is that we are tired of the same old cliche'd horror movies over and over and over...so in the end, the gods rise to destroy everything in hopes that the horror genre will "wake up," so to speak, and change things.
Great, great movie. There are some good discussion threads about this one all over this board if you can get the search function to not be an a-hole.