For reasons best known to himself, one Paul__RB oops sorry I mean Paul__B has taken this into a PM chain, so in the interests of clarity here's his main point:
No board since the Arduino Diecimila has had a resistor in series with pin 13. Earlier boards did, and in fact this was a design fault that rendered the pin unusable
I take that to mean I'm right in what I said about the Uno R3, where the resistor protects only the built-in led. Here's a snip from the Uno R3 schematic.
