the MegaTexture technique by John Carmack. :)

altough the idea isn't really groundbreaking, you at least have to find a
speedy way to implement the streaming, unless you're generating the textures at runtime (farbrausch, anyone? :D)

I think the biggest positive on having an engine that streams the texture-data is that - providing it works *g* - you never ever have to think about handling texture maps again, because you get vram-size-independency. :D