In last week's Famitsu, Tetsuya Takahashi revealed some plot info on his current project, Xenoblade, some of which details the purpose of those two monolithic creatures currently displayed on the official website. Apparently these two gods engaged in battle 10,000 years ago and resulted in two corpses thereafter and Xenoblade's story begins on top of those corpses. This might seem absurd but rest assured Takahashi has a "methodical" approach to this "madness" (so to speak):
It seemed like it would be enjoyable to adventure on top of the body of a giant god.
The game world is set on top of these two godly corpses. An ecosystem with two opposing residents inhabited by each corpse; one is an organic species, the other is mechanical. This will play a major role in the game world as they're at war with each other (i.e. the mechanical residents threaten the peace of mankind/organic species etc). How interesting, there is more info over at adriasang.