For unlimited terrains, you need good tiling terrains (33X33, to give a good variety of plains, mountains etc) in a 9 X 9 grid and lock your player in the center then get all other elements to move around the player instead of the player moving.

Alternatly as stated above, in one of the AUM series (after 30 I think) is a tiling terrain that does a similar thing except player moves instead of the terrain.