Dead right Mike, the motor was indeed under no load when tested and showed little sign of speeding up or slowing down when applying the pwm to the enable pin.
This led me to research the subject a bit and I discovered the "locked-antiphase" method of pwm control which runs an inverted signal of the pwm signal together with the pwm signal to the two signal/direction pins.
Apparently applying pwm to the enable pin is called a signed magnitude type pwm.
Best description I could find is here http://www.openservo.com/forums/viewtopic.php?t=142 (check the quote on the first post) ..