Turns out it was a bit more difficult than I thought.
I tried at first with an Infrared rangefinder sensor however this proved to be too erratic probably due to the shiny surfaces on the front of the vehicle. Also the narrow beam of the IR sensor made adjusting the sensor problematic. I then went with a Maxbotix ultrasonic sensor which has a wider beam and was not sensitive to the light refracting surfaces.
I didn't include much as far as step-by-step, I've included some photos, a circuit diagram and the Arduino code i used. With this information any Arduino hobbyist should be able to make a similar sensor. The case I used was an old hard drive box from a Compaq laptop, of course any suitable sized project box would work fine.
The board I picked was a Diavolino from Evil Mad Science. I pickted it because it was inexpensive and cool looking. The board doesn't have a USB controller for programming so I bought the Adafruit FTDI Friend. This makes projects less expensive since I can buy an Arduino board without the controller then reuse the programmer for different projects.
I created a Youtube video showing a quick overview of the parts and showing the sensor in action.
Please post questions or comments below. Have fun!