Well, we already have Maya '09 Unlimited. I see a lot of problems with using it to build the whole world though. Namely, it doesn't have the tools that make a world builder a world builder. You can't paint textures onto terrain (afaik), and placing 3D objects will end up making it part of the terrain. Of course, we could do object placement in WED, but we do need to be able to manipulate terrain and place objects in the same program. This will be crucial when building towns, cities, roads, etc. We'll be needing to modify/flatten certain parts to make buildings "fit", and we'll need to be able to make trenches, foxholes, etc in the actual world.

Btw, we aren't planning on making different "levels", but instead, one monstrous world. So practically everything will be built of models, not blocks. We're going to have to break up the better part of North Africa into segments, construct it, edit it, and reassemble it the engine with proper coding techniques to make a seamless world.