Actually, the point of Megatexture technology is to create huge expanses of terrain that don't have repetitive texturing, because that's obviously a huge problem with today's games. It's a brand new thing though, and I can't imagine why anyone in their right mind would think that Half-Life would be able to handle it. I can't personally see why you would even want it, since you can barely make convincing outdoor areas as it is, what with the lack of displacements, 3D skyboxes, and enough real estate to put it all in.
@Muzz: I think what Urb was asking was whether or not the maximum texture resolution for models was 512x512.