IIRC, I saw this first on Gameinformer, though I saw it two other places as well yesterday. Seeing it definitely made me want to share it here. As always, click to make bigger.
This piece is done by AndyDaroo. You can find this and more of his artwork on his DeviantArt page. The only other Halo piece he has at the moment is this Master Chief vs Scarab.
Those are absolutely awesome!
Props to the artist!