I believe that there a bit of confusion.
For large 240V appliances (ranges and dryers), old code permitted the use of the use of the same conductor for both grounding the frame and neutral return of 120V loads. This is _not_ necessarily a violation.
I don't know the details of older code, but in the 2002 code, there are specific descriptions of when such a circuit make be continued to be used. In particular, the ground/neutral wire must either be insulated, or the cable must be service entrance cable. Type NM with two insulated conductors and a bare ground would be explicitly prohibited. But if the install was done using Type MC cable with an insulated ground, then my read says that it can still be used.
All _new_ installs must correctly separate ground and neutral.
I don't see how it could ever be appropriate to connect a three conductor cable to a 4 slot receptacle. Ground and neutral would be shared, but this would be hidden back in the wall.
-Jon
