[BradAMiller]

Here's an excerpt from the Users Guide that will publish in an update in the next day or so (the formatting will be better in the document)...

PID controllers are a powerful and widely used implementation of closed loop control. The PIDController class allows for a PID control loop to be created easily and runs the control loop in a separate thread at consistent intervals. The PIDController automatically checks a PIDSource for feedback and writes to a PIDOutput every loop. Sensors suitable for use with PIDController in WPILib are already subclasses of PIDSource. Additional sensors and custom feedback methods are supported through creating new subclasses of PIDSource. Jaguars and Victors are already configured as subclasses of PIDOutput, and custom outputs may also be created by sub-classing PIDOutput.
The following example shows how to create a PIDController to set the position of a turret to a position related to the x-axis on a joystick using a single motor on a Jaguar and a potentiometer for angle feedback. As the joystick X value changes, the motor should drive to a position related to that new value. The PIDController class will ensure that the motion is smooth and stops at the right point.
A potentiometer that turns with the turret will provide feedback of the turret angle. The potentiometer is connected to an analog input and will return values ranging from 0-5V from full clockwise to full counterclockwise motion of the turret. The joystick X-axis returns values from -1.0 to 1.0 for full left to full right. We need to scale the joystick values to match the 0-5V values from the potentiometer. This can be done with the following expression:

(turretStick.GetX() + 1.0) * 2.5

The scaled value can then be used to change the setpoint of the control loop as the joystick is moved.
The 0.1, 0.001, and 0.0 values are the Proportional, Integral, and Differential coefficients respectively. The AnalogChannel object is already a subclass of PIDSource and returns the voltage as the control value and the Jaguar object is a subclass of PIDOutput.

	Joystick turretStick(1);
	Jaguar turretMotor(1);
	AnalogChannel turretPot(1);
	PIDController turretControl(0.1, 0.001, 0.0, &turretPot, &turretMotor);
	
	turretControl.Enable();  // start calculating PIDOutput values

	while(IsOperator())
   {
		turretControl.SetSetpoint((turretStick.GetX() + 1.0) * 2.5);
		Wait(.02);		// wait for new joystick values
	}

The PIDController object will automatically (in the background):
• Read the PIDSource object, in this case the turretPot analog input
• Compute the new result value
• Set the PIDOutput object, in this case the turretMotor
This will be repeated periodically in the background by the PIDController. The default repeat rate is 50ms although this can be changed by adding a parameter with the time to the end of the PIDController argument list. See the reference document for details.