kg4wsv:
Low (dc) resistance isn't the issue: you can actually short your avr's pins without damaging them.
You've done this?
I guess Atmel doesn't know what they're talking about when they give the current limits, both per-output and per-device, in the datasheet.
The simple fact is, you kill the arduino (actually the ATmega) by trying to draw more current than the chip is designed to handle.
-j
A short will either shut the regulator down for the DC input or open the PTC (fuse) for the USB. An overload could easily destroy the outputs. This is one of the reasons I cringe when I see people recommend that an LED being directly connected instead of through a resistor as an LED (or any other diode) is a short that just opens up below a certain voltage.