dhenry:
I beg to differ.
Your analysis is correct, if the motor is held motionless. Your analysis is wrong, if the motor is allowed to move.
I gave two examples: one with the motor held motionless, and one with it rotating at a constant speed and thereby producing a constant back emf. Would you care to explain what you think is wrong with my analysis of the second example? I am assuming negligible inductance (as Mike has pointed out), and I am also assuming that the back emf is constant, whereas in practice it will have a ripple at a frequency proportional to motor speed.