Jeff,
Metal conduit has historically been used as a ground path. It is also notorious for causing problems when not properly installed. Loose joints, connectors or lock nuts, not making good contact, lack of bonding bushings or bonding lock nuts, can all contribute to an ineffective ground path.
Corrosion and rust can also negate the metal conduit as a ground path.
Although more expensive, many contractors will install a ground conductor sized from 250.66 (service) or 250.122 (sub panel) as a means of insuring an effective ground path.
Alan--
