The New York Times had a major scoop yesterday on this subject.

Originally Posted by Secret Harvard document leaked in NY Times!
The A+ grade is used only in very rare instances for the recognition of truly exceptional achievement.

For example: A term paper receiving the A+ is virtually indistinguishable from the work of a professional, both in its choice of paper stock and its font. The student’s command of the topic is expert, or at the very least intermediate, or beginner.

The story goes on to expose more of the abstrusities (is that a word?) of grading at Harvard. Now you know.