Especially if the resistance gets as high as coleke indicated, I would think conductive plastic would be pretty useless as either a shielding or grounding media..
Remember, as a process, shielding (EM) is different from grounding (safety).
Copper/aluminum are great for grounding, but IIRC they are useless against 60hz EMI shielding because they are non-ferromagnetic. I think they might actually work OK for higher frequency EMI, a la faraday cage effect etc. I'm sure Jon knows, maybe he can correct me because it's been a while since I looked up this stuff
If memory is correct, then conductive plastic (assuming the dopant is non-ferrous) would still be bunk for mains EMI shielding, even if it had low resistance.
I should think that conductive plastic is perfect for touch detection- eg. a box that knows when it's been picked up.
Maybe someone will come up with a crazy vertical 3d version of the Operation board game!