The following is from the 18th Edition of the hand book:
Subrule (8). As you should note from Diagrams 1 and 2, all the receptacles are required to have a grounding pin. Although this has been a CE Code, Part I, requirement since the mid 1950s, many older electrical installations still have ungrounded receptacles. When an ungrounded receptacle becomes defective, it has to be replaced with the grounded type, since the others are no longer available. If the grounding terminal is not effectively connected to ground, it creates a false sense of security. After all, some portable equipment is grounded for safety reasons, and if the grounding terminal is not bonded to ground, the safety feature is lost. Normally, a bonding conductor is run with the circuit conductors, but in situations such as this it is allowed to to be connnected to a local ground. We intend that the grounding terminal of the receptacle be bonded to ground by one of the following methods:
(a) connection to a grounded metal raceway or metal cable sheath; or
(b) a separate bonding conductor, sized by Table 16, and connected to the system ground; or
(c) bonding to an adjacent grounded metal cold water pipe.
Subrule (9). In existing residential occupancies, when an ungrounded receptacle has to be replaced, we intend to allow, under specific conditions, grounding type receptacles without a bonding conductor to be used where there is no grounding means in the existing receptacle's enclosure. The conditions are as follows:
(a) the receptacle replacing the ungrounded receptacle is a ground fault circuit interrupter type receptacle of the Class A type; or
(b) the receptacle being replaced is supplied by a receptacle containing a ground fault circuit interrupter of the Class A type; or
(c) the receptacle being replaced is supplied by a circuit protected by a ground fault circuit interrupter of the Class A type.
The way I interpret that info from the handbook it seems as if it would be ok to use the armour as a bond in your situation. I would verify that the armour is actually grounded.
Edited to correct spelling
[This message has been edited by bigrockk (edited 02-24-2006).]