All objects cast shadows on themselves unless they are set to "Shadeless". You might think that this is still like casting shadows, but if so where does it get the light from?

The point here, though, is that a "shadow pass" probably shouldn't include the shadows that an object casts on itself -- at least in the way provided in the example blend. The dark faces are receiving shadows essentially because they are pointed away from the light -- on the dark side of the moon, as it were. (An example of when you would want it to cast shadows would be a character's hand dangling in front of his leg, which would cast a shadow on the leg.)

The dark side of the moon is made dark by the diffuse shader already. So if the shadow pass is ALSO returning a dark color on the dark side of the moon then it messes up the composite, as the diffuse [obviously] isn't adding any shadows cast on the floor. So I've got 2x darkness on the side pointed away from the light, and 1x where the shadow falls on the floor.

What you mix up is diffuse shading - which only evaluates light when a normal points to the light sours - and shadow.
Shadow is based on occlusion of faces, the backside of spheres or objects are not shadow, thats just diffuse shaders at work.

Ton: If "the backside of spheres or objects are not shadow" then are they supposed to show up in the *shadow* pass? That's what has me confused here..... I've identified a better way to isolate this, and will float it around blenderartists before resubmitting. Thanks!