If the Harvard economics major requirement of B- grades in the intermediate economics courses is not for capacity control reasons (as high grade/GPA requirements for majors at many less well endowed schools are for), then it may be an implicit admission that grade inflation has resulted in a C grade no longer being an acceptable passing grade.
http://www.gradeinflation.com/Harvard.html indicates that the GPA overall at Harvard was 3.45 in 2005.