Why do Arduinos ship w/ 16MHz crystal instead of 20MHz?
I'd buy into the answer on the Arduino Forum: 


  The original ATmega8 Arduino ran at 16MHz, which was the top rated
  clock speed for the ATmega8 cpu used. When "upgraded" to ATmega168
  (with a 20MHz top cpu speed), the clock was left at 16MHz (probably)
  because the designers thought that more people/code would have
  backward compatibility issues with a new clock rate than would benefit
  from the extra 25% cpu performance.  I certainly think they were
  right...