The Arduino Nano is a small, complete, and breadboard-friendly board based on the ATmega328P (Arduino Nano 3.x). It has more or less the same functionality of the Arduino Duemilanove, but in a different package. It lacks only a DC power jack, and works with a Mini-B USB cable instead of a standard one.
The Arduino Nano can be powered via the Mini-B USB Jack (5V) connection.
The Arduino Nano accepts the 7V to 12V input power from the Vin pin (pin30).
If you want to supply regulated power, then a 5V regulated adapter needs to feed the +5V pin (pin27) instead.
The power source is automatically selected to the highest voltage source if you input multiple power inputs.