In general, program managers are responsible for requirements development and overseeing the technical progress of a program, if the program requires development. He or she is ultimately responsible for the cost, schedule, and effectiveness of the system or service that his program office has been chartered to deliver.
DHS is similar to DoD, so we have engineers, logisticians, IT specialists, etc. that are all part of our PMO's. A PM doesn't necessarily need to be an expert in these areas, but he or she needs to know enough where they can manage these technical specialists. In a perfect world, a PM would rotate through all these different specialties to learn how they operate.
Incidentally, if you look at the competencies that FAI published over the summer for program managers, the most important trait is "leadership." One thing that I've seen at CBP (not sure how widespread it is throughout the federal government) is that employees aren't eligible for leadership training until they're GS 14's...and by that point, it's generally too late. Again, in a perfect world, leadership would be considered part of the "developmental training" for lower GS grades (9 through 12). I think the time investment would give us much stronger PM's.