I googled it quick and found that other people have a bunch of the same problems. After some tweaking, I came up with this workaround, and I really hope this feature is implemented easier in future versions of Blender.
Have your object that you don’t want to cast shadows on layer two, with a lamp on it that’s set to illuminate objects only on the same layer and not cast shadows. Also make sure your object on layer two is set to not receive buffer shadows.
On layer one, have another lamp in the same spot as your first lamp, but have it set to have ray shadows. Make it in a light group called castshadow. On the object you want to be the mirror and cast shadows itself, set it to only receive shadows from your castshadow light group, and make it exclusive so that light group won’t affect the first object.
Your scene shouldn’t be illuminated much, so if you add a spotlight with a buffershadow, then it will cast a shadow on the scene, but not on your first object because you deselected buffershadows.
If you followed that explanation, then you’re a genius. I don’t think I could follow my own explanation, so I’ve attached a blend file and a picture of the finished image.
By the way, that square in the reflected red plane is another reflection of the mirror.
mirrorproblem.blend (133 KB)