The jacket of standard sriral wound MC cable is not sufficient to provide grounding without supplemental grounding conductors.
See 250.118(11).
As far as your installation, there is an assortment of special MC cables available. This includes those that have additional grounding conductors.

If you have a handbook, the commentary following 330.108 explains this situation.

[This message has been edited by Redsy (edited 09-17-2002).]