Crank arm length has always been a controversial topic. When I rode in grad school in the mid '70's, 165mm - 170mm were the norm. Now the norm is 170-175 for men's road bikes OEM. Shimano Dura-ace cranks (Hollowtech) are currently available 165mm - 180mm in 2.5mm increments. General rule of thumb is the taller the rider, the longer the crank arm length. You are looking at spending big bucks to change crank arm length in the aftermarket. As a result, most riders accept the length that comes with their bike. The question as to length comes up most commonly when a rider is doing a custom bike (either with a custom builder or building a bike up from a frame), or when a rider has a problem (like knee pain or back pain), or when a rider just wants to experiment (usually a racer with access to enough other bikes/crank arms of different lengths with mechanical expertise to do the swapping).
Of interest, a lot of the riders that I know are trending to go shorter (if they have a choice: replacing a crankset or getting a new bike) as they age (50's +). Easier to spin and less stress on the knees.
Lennard Zinn recently wrote on the topic in his column on the VeloNews site:
http://velonews.competitor.com/2011/06/bikes-and-tech/technical-faq/technical-faq-with-lennard-zinn-when-it-come-to-crankarm-length-no-easy-answers_178528FWIW, I am 5'4" and spin a 165mm crank.
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