Trying to get a light to cast light and shadows only on a certain layer using the "layer" option in the lampbuttons. The result: Only objects in that layer are lit (good), but other objects in the scene not in that layer are casting shadows (bad).

Yup... its a known defect. Always check the "Bug Tracker" (link is in the side navigation). I added that defect and they've marked it. So hopefully it will get looked at for the next release.