I need to put an addendum on that last post, with new stuff I've learned since then.
If you take a lead off the sensing/ground lead, and take it to an analog input pin, you'll get a voltage across the resistor if it's there. This can be used to provide feedback on how much power is being pulled by the motor, and PWM can be adjusted to provide constant current depending on the conditions.
Most people will be more worried about the speed of the motor however, which isn't consistently available through the current level. Current control can tell you if the motor is stalling, but generally if you have a reasonable method of measuring speed, you'll be able to tell the motor is stalling that way.
You don't need a resistor unless you're messing with the current sensing, but if you are, the wattage rating on the resistor needs to be selected properly(probably over a watt), and the resistance needs to be chosen to make sure the voltage being sent to your sensor pin doesn't fry it.