Is this actually true? I don't attend Harvard, but I do attend a private university in the top 10 of the US News and World Reports rankings, and I don't believe there is a single CS course taught by a grad student (although some are taught by "lecturers". Some of the classes from the core curriculum are taught by grad students, but that's not actually necessarily a bad thing. Both of the grad students who have taught courses I've taken have been excellent teachers, which can be much more important for a teacher of introductory calculus or such.