I am not sure how they did that, but I would think that making a sphere or something in a 3D program like blender and rendering it that way would work. I'm curious as to how it was done too ^.^

I have seen that suggestion before, but I haven't seen it explained much further than that, e.g. the scale of the sphere in comparison to the camera, orientation (I realize it's one camera for each of the 6 directions, but what rotation should each camera itself have), and the trick of setting up the sphere/semisphere to be rendered.

It was rendered out as an evironment-map(cubemap/skybox) in 3Dsmax.

I can post a tutorial if anyone would like to know how.