On a related note (sorry if it's been brought up before - I haven't done a forum search on the topic)... how are these things legal?
If you have an ungrounded 2-prong receptacle, then there's probably no EGC present in its box. So if you plug that little dingle-wizzy in, and install the receptacle trim screw through the little green metal tab, you're still not grounding anything.
I imagine the instructions that come with one of those things say "Do not install on receptacles with no EGC present" or some such, but every time I see these things at the hardware store, they're just piled into a bin with no packaging or instructions or anything.
Again, sorry if this has been discussed previously. Just baffled by these things.