I would use a microphone because unlike a piezo element the mic has a much flatter response as the frequency is varied. The signal would need to be amplified and then I would run the amplified signal into a comparator the output of the comparator could be fed into a digital input on an Arduino. The Arduino could measure the time between signals coming and get the frequency and then use a look up table to determine which string got plucked.
This is not as hard as it sounds. One issue that you will have to watch out for is harmonics of the fundamental freq of the string, the string will not give a pure tone but there will be signals at multiples of the fundamental freq. present. By setting the threshold correctly on the comparator you will be able to filter out the harmonics.
wade