The LED on current Unos and Megas (since 2010) is driven by a comparator connected to D13 and requires next to no current to drive it. The internal pullup resistor works just fine.
Other models have varying resistor values driving an LED directly, so the resistor/LED may present a load of 2.5mA to 8.5mA to an external driver (assuming Vf of 2.2V for the LED and 330 to 1K series resistor) and the note in reply #5 may come into play.