Clock-watching Federal employees is most certainly a fallacy.
Working in government for at least 8 years (4 being as a current Fed, the others as a contractor), I've seen the majority of people I know as dedicated employees (even the contractors usually) that are willing to stay late, work on the weekends, or willing to take on the job of two people just to get the work done. Sure there are some who will try to skirt by on their 8 hours and nothing more. But I believe those are fewer than one would expect.
I was raised to do what it takes to get the job done...I think many Gen Xers are the same way. People often say that moving up in your career means knowing the right people...but that's only half of it. The other is not just doing "what's required," but instead doing "what's needed." I do what's needed and I believe many Federal employees are the same.
Plus, many of us [Feds] have some type of smartphone, and access to work from home. We're always connected. I can tell you that when I have nothing else do at home, I'll do work. I'll check email, respond back and answer what needs to be answered. If I see something that I can do then, I'll log in and take care of it. It helps to keep my life sane when I return the next day, or on a Monday morning. If you added up the hours that I work from work, home, and mobile during a 2 week period it would certainly be more than the 80 hours required. Does it bother me? Not at all. I do it because I care about my job, I care about the work I do, and I have a responsibility to organization and the public. I do what's needed, not what's required.