John Carmack is developing a new technology called MegaTexture:
John Carmack at id Software devised the MegaTexture technology as a way to generate and exploit a single huge, unique, untiled texture to cover the entire landscape of a map. This represented the first of several significant advances for the Doom 3 engine; for example, the MegaTexture technology has a tool suite called MegaGen, which really helps our Level Designers generate the initial map layouts. MegaGen can automatically distribute materials such as grass, sand and rocks across the landscape, based on altitudes and the angles of incline. MegaGen makes moss grow up the steeper slopes and cling to rocks, grass grow in the flatter areas, and sand and snow gather appropriately in the crevices between rocks. Our Artists are then able to paint additional fine unique detail such as cracks in road surfaces, or they can texture modeled elements such as shell and plasma blast craters in the terrain..
It all sounds pretty mega. More info over at ShackNews.