I don't think that is true everywhere when not on the clock.
http://en.wikipedia.org/wiki/Duty_to_rescue
Slight clarification here: I was always taught that once you've been identified as being some sort of trained individual, then you do have the legal responsibility to act.
So, if you have a big Star Of Life on your window, or wear your "Mayville EMS" hat or something, then you actually do need to stop and render aid. A big reason I don't do that much anymore.