guckst du hier [link]http://www.arduino.cc/en/Main/ArduinoBoardDuemilanove[/link]
The power pins are as follows:
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.
also entweder am VIN anschließen oder am power jack

was passiert wenn dus am 5V anschließt?
keine ahnung, ich würds nicht probieren

lieben gruß