While the definitive manual is yet to be written this is a good start to programming on the Arduino:-
http://www.lulu.com/items/volume_63/1108000/1108699/4/print/ARDUINO_NOTEBOOKv6.pdf
However on page 29 where it says:-
The resistor may be omitted on this pin since the Arduino has one built in.
Do not believe it, you need a resistor for an external LED