Spotted in the MAKE Flickr pool, nickjohnson writes:

I made a strange piece of modern furniture with concrete and LEDs. I built this by first constructing a mold out of MDF, and by embedding LEDs into the surface of the mold. The control/batteries are located behind a panel on the bottom, and all the wires are routed through the concrete.

Check out build photos in his flickr set.
More:

• How-to: Cast concrete planters using packing inserts
• Concrete Lamp