Hi GL,

It's an easy trick. There is actually no lights in the scene.

Use a bi-gradient background with white at the top and a mid gray towards the center. Enable Global Illumination and Ambient Occlusion and your done. I've already trashed the file I made the image from or I'd give the exact settings. :-P  For best results, you'll probably want to also enable "better edges" and boost the photon count and fidelity up to prevent any "chunkies" in the shadows.

Mark