Using Blender Internal, is there a way to make light pass through a solid object, such as a cube, as if it wasn't there? The cube of course would still be rendered.
Go to the Properties panel > Materials tab > Options menu and deselect Traceable. Now, if, for example, you had a Cube with a Point Lamp inside, the point lamp would still affect the scene's lighting. You will, however, also have to have lamp(s) outside the cube to light it.
Also, consider looking at this .blend file. The cube in the center is the one with the lamp inside.