It depends on the iPod model, I have a fifth gen nano that went through more abuse in the first month than most iPods go through in a lifetime. The left and right output on mine have been shorted together and both work just fine, they seem to have an internal resistor for protection. I also measured the output with an oscilloscope and the voltage managed to reach 2.5Vp but never dipped below ground. I can't say that this is true for all iPods, I'm sure they are all different.
And for clarification, the list of deaths from my iPods:
1st) iPod Nano G2 --- Manor of death unknown, cause of death- faulty wiring in a homemade charger
2nd) iPod Nano G5 --- Manor of demise- faulty amp feedback into headphone jack, not dead just useless
3rd) iPod Nano G5 --- Manor of death- back to the charger again
4th) iPod Nano G5 --- Still going, currently in the process of dying from old age. It was a refurbished one and the battery has started having a hard time holding it's charge.
The G5s are starting to become hard to find now so I'm currently babying this one.