These two concept screens were spotted by The Little English Halo Blog, and are featured in the Halo 4 Premium Theme. There's no word on exactly what these two screens depict, but their beauty and scale is incredible.
The first screen depicts a lonely Forerunner structure that is glowing with a neon blue Tron-esque light with the Chief standing center-stage. While the second (and more interesting) screen shows us an upside down city-scape hanging over a mountain range.
We're not sure what's going on there, but it could be some kind of shield world, with buildings pointing down from the shield layer onto the surface. Either way, we're excited for the next Halo game to dig into some of the awesome lore that exists in the peripheral fiction.