Following up on Nathan's post, I remember that my statistics teacher at INSEAD (a business school) gathered data on student's grades and their salaries 5 years after graduation. He then ran a correlation test, and he found that there is a statistically significant correlation... negative! In other words, on average, the higher the grades, the lower the salaries after 5 years.
Possible explanations? I'll give mine: students with higher grades tend to be more risk-averse, and risk brings big rewards (for instance, self employment, etc).