Good sam laws vary by state, and are not national. In many states, you are covered, with the STIPULATION that you respond to the level of care you are trained to, given the situation (obviously, you won't have a crash cart, meds, and IVs with you). Some states do differing laws which could be interpreted as having to respond. Some agencies that employ healthcare workers have stipulations on whether or not to respond, and personal insurance policies vary here too.
Good sam laws, as many can tell you, do NOT prevent you from being sued, they offer limited protections.
There are some gray areas. Identifying yourself as a healtcare provider is like saying, "I'm trained to help, can I help you?" . Many Newbie EMTs and other healthcare providers like to have EMS bumper stickers, or shirts identifying them as healthcare providers. In some states, this can be seen as identifying yourself to the patient as a healthcare provider, and thus would require response... WY and CO have had some court cases with EMTs who have either been wearing EMT shirts or having EMT bumper-stickers, and didn't respond...
I actually worked for an industrial clinic as a medic. We had a worker get in a severe car accident on the way to work, only a mile away. The closest town was 22 miles. This happened around 3:00pm, the time our facility physician leaves. He was aware of this, and rather than stay and treat the patient, or respond to the accident, he took another route around the accident, ensuring that he would not have to render aid. Oddly enough, he is also the medical advisor for the ambulance service that responded (the nearest town). No action was taken, as in my state, response by a healthcare provider is not required.