DrOb:
There's a couple of Application Bulletins on the site of your motor supplier that help explain (essentially what everyone else above has mentioned - the current and back EMF can damage your arduino):See driving motors with PWM section here:
AB-012: Driving Vibration Motors With Pulse Width Modulation - Precision Microdrives
Thanks for the link! I apologize if this is a naive question, but if I use separate power supplies for the Arduino and the motors, will that not be enough to prevent issues with noise and back EMF?