I've just begun to integrate HippoCampus into my American Government course. For now, it is identified simply as supplementary readings, although I am seriously considering using HC in lieu of a textbook for basic knowledge, and building other supplementary readings, projects, and activities around it. I already have numerous such items on my web page. I welcome any comments or suggestions regarding this approach. For details, see the following:
http://sp.stmarysannapolis.org/highschool/klatham/courses/ag_assignments.aspx