A background sounds like the most boring part of a scene, it doesn't have to be. In Chapter 6, Creating 3D Effects in 2D, we touched upon this when we looked at ParallaxBackground and the similar AutoParallaxBackground example. These types of backgrounds aren't merely flat backgrounds but have multiple layers, which creates the illusion of depth, as the background layers in the front will move faster than those behind them.
For a world built using 3D resources, we would still be using a flat texture as the background, much like the way the world in a 3D game is enclosed by a skybox or similar. In essence, this doesn't change much compared to the 2D scene, other than the use of different object classes, as we saw in Chapter 2, Replacing 2D Sprites with 3D Models.