From the Bungie Update:

"over a period of several months and literally hundreds of files, we have the final Halo 3 logo. We hope you like it and if everything goes according to plan, you’ll see a LOT more of this in the months ahead!

Looks like the color and texture of metal on the underside of the Halo rings; although at a distance, the colors also look like an Earth-like world seen from a distance, the mix of blue, green, and white.