Look in the WIP board for BlitzTerrain and XenoPhysics. These are going to be very cool for games requiring terrain, especially large terrains like a flight simulator. The first release of BlitzTerrain isn't going to include terrain streaming I don't think, but when that feature comes out that's what you're going to want. Endless terrain...
Skyspheres are pretty easy to understand. Make a very large sphere with a negative size. The negative size puts the face that gets textured on the inside of the sphere. Now just texture it with an appropriate image. Skyboxes work the same way, but require 6 separate images that you put on each face of the box.