Where does light come from in a Cycles scene without lamps? Using the Blender Render, if you have no lamps, the objects are black.  With Cycles Render, even without any lamps, there is a small amount of light.

Here is a render of the same yellow monkey without any lamps present using the two render engines:



Where does this light come from?  Can you turn it off?
In cycles the World emits light. You can turn this off by setting the world color to black, or setting the Strength to 0 in Properties > World:



If you still want the background to appear as a color besides black, you can make the world emit light only for the camera by mixing two Background shaders based on the Is Camera Ray output of the Light Path node:



Result: