Is it possible to use 1 power source to power an arduino duemilanove and a dc motor? My goal is to use a wall wart that is either 9vdc or 12vdc and power up the arduino while powering up a motor that uses a relay with the arduino to control it. I know when I connect the motor directly it resets the arduino. I didn't know if I could just put a large capacitor in parallel to help buffer the motor distortion, or if there will still be problems. Any advice?
If your 9V power supply has enough power for both you can use it for both. Connect 9V to the power jack or Vin(+) and Ground(-).
If your relay coil requires more than 5V or more than 30 mA you will need a transistor to drive the relay. In that case you could use the transistor to drive the motor directly and skip the relay. This would allow speed control if you connect the transistor to a PWM output pin.
Do a Google search for Arduino Transistor to find out more about driving motors and relays with transistors.