Initially it was just an engine I made up, but like most of my concepts I had no game to go with it.
Mind you its not true 3d, It basically works like Terragen you create a height map, and from that you can make or apply a texture to the mesh that the height map generated.
There are some limitations, you wont be able to go over 50x50 grid size, that's a hardware limitation according to Ashley, but like I said the mesh's are fairly scale-able, so your mesh, or texture, wont have to be 50x50, but the height map will. That means you have a couple options for making bigger maps, you can either string a bunch of smaller mesh's together, or just one big one, or even something in between.
There's still a bunch of stuff thats' possible that I haven't tried, like deformable/animated terrain, and even random terrain maps.