From the fine Digital Pins Tutorial we have this quote:
Short circuits on Arduino pins, or attempting to run high current devices from them, can damage or destroy the output transistors in the pin, or damage the entire Atmega chip. Often this will result in a "dead" pin in the microcontroller but the remaining chip will still function adequately. For this reason it is a good idea to connect OUTPUT pins to other devices with 470Ω or 1k resistors, unless maximum current draw from the pins is required for a particular application.
Does this mean that an OUTPUT pin should never be shorted directly to 0v or that an OUTPUT pin should never be shorted directly to 5v? I've seen both types of shorts in various tutorials, usually via a SPST switch.