To add a bit, the earth is somewhat conductive in the local sense, and rather conductive in an overall sense, but never used as the only pathway for neutral or fault currents.
The purpose of bonding, and grounding this bond, is to minimize voltage differences (potential) between different conductive surfaces with which one might make contact.
The metal cabinets and the earth are at the same potectial (even if not zero volts) so someone standing on the earth and contacting an energized case doesn't conduct electricity.
[This message has been edited by Larry Fine (edited 06-12-2005).]
