Seen recently on Low End Mac:
Although earlier models had less hard drive bays…
“Less,” much like “amount,” is used in comparatives that cannot be quantified. For example, “Geroge W. Bush has less intelligence than your typical ironing board” or “George W. Bush’s average speech exhibits a miniscule amount of public speaking ability.”
“Fewer,” like “number,” is used to make quantifiable comparisons. “George W. Bush has fewer brain cells than your typical head of cabbage, and he only uses a small number of those, particularly when making speeches.”
See also Grammar Slammer’s explanation.