Well my "off service" rotations were before the 80 hr restrictions, so yeah, I did work more than that. In our program, we were still required to take in house trauma call while on off service rotations and of course, we did not go home post-call.
The "off service" residents were all Prelims on GS as PGY-1s (they did not rotate on GS otherwise) so we were not replacing them per se as we were not interns on their service, although I recall their being some rules about when we could be on those services.
Vacation time for our program was scheduled at the beginning of the year; there were months when you couldn't take vacation (June, July, December and right before our ITE). Some rotations would also not allow vacation to be taken - can't recall which those were but in general, you didn't know what your schedule was going to be before putting in vacation request (ie, you just picked the week and the Admin Chief worked the schedule around those requests).
FMLA? I don't know...it wasn't that much of an issue in my program. I'm the only one I know who took it and people were pulled from the lab to take call in my stead.
Sending people home was dependent on the Chief. Some were sympathetic and others were not. But it was not ok to "call in sick"; you had to show up, the Chief had to assess the work flow, see how many people would be available to cover the work/cases, etc. I had some Chiefs/seniors who would send you home routinely but many others would just find an empty bed for you in the call room, start and IV and periodically check on you to see if you felt better. I remember one Christmas Eve when the PGY-4 and I were on trauma call, taking turns in puking in the call room (hi Rob!); short staff, no one else to cover, so we sucked it up. Despite what one poster writes above, this was not about ego, it was about manpower and doing the job you were asked to do. Believe me, I would have much rather been at home those times.