You've probably resolved this, but I have an unanswered post and it's annoying!
As I understand it (I'm relatively new, too) it's a very safe selection. With 5v coming through the circuit from the Arduino and the LED itself using up approximately 2.2v, the resistor has to use up 2.8v in order to protect the LED from burning out (the maximum continuous current through the LED being 30mA). A 10,000 ohm resistor therefore allows a mere 0.28mA (Ohm's Law: Current = Volts/Resistance; 2.8/10000 = 0.00028. A 1,000 ohm resistor would have allowed 2.8mA (still very safe, and a bit brighter). Even a 100 ohm resistor would have been within 30mA (and the LED would be much brighter). It's worth tinkering to find out!
