Yesterday's Scientific American addressed in a blog one of my linguistic pet peeves: the increasingly common failure of speakers to distinguish between meaning and usage of "less" and "fewer."
It seems mighty simple to me. If you count it, the reduction in number is referred to as "fewer." If you measure it, the word is "less."
Scientific American explains the distinction as deriving from the mathematical concepts of "continuous," which applies to things you measure, or "discrete," applying to things you count. Here is the article.