I ask this question because the guy I play against used that reasoning to wipe out my assault squad, simply because my chaplain's ( jump pack model ) thumb was visible.
edit : His reasoning was that the rulebook was specific : head, torso, legs, arms, and that the thumb is attached to the hand, and thats attached to the arm.
I debated this , simply because of the fact that that it wasnt the arm, it was the thumb, and there is a further exception to the rule in the fact that wings and tails do not count as part of the model. Even though they are "attached" to the torso.