Ok so basically I am trying to power a motor, I plan to use the vin pin (which will be 12v), what happens ifi ground this pin with little added resistance?
Will it damage the pin or the arduino in any way?
Sorry I'm totally new to this, I mean like "omfg new".
Thanks 
p.s. I've got the Duemilanove
If you are putting power into the 2.1mm power connector, prolly not a good idea to ground the Vin pin, you'll end up with smoke release as this would be a dead short to ground on an unfused power source...
Schematic for your Arduino... http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf
VIN. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.
You can apply 7-12V DC to this pin to power your Arduino or use it as a power tap for up to possibly about 500mA when applying power to the 2.1mm power jack. The limit isn't mentioned on the hardware page. http://arduino.cc/en/Main/ArduinoBoardDuemilanove Your statement "little added resistance" taking into consideration Ohm's law for 12 volts in is 12V/0.5A =24 ohms minimum to limit it to 500 milliamps.
The trace leading to the pin isn't that wide, so I'd be happier to be pulling less than 500mA off the pin and would personally power the motor with a separate supply with common ground to keep motor brush hash out of the Arduino, they don't like that kind of noise on their power supply, unless you use a couple bypass capacitors to decouple the power supply.
If you want to power a motor you will need to switch it with a transistor or relay as well asusing an external power supply. See:- Motors 1