Captain Stable, I do get your point, but the fact is, that there never won't be a determined thin line. If there were one, then EVERY decision could be devided dowm into a YES/NO one. YES/NO is in Computer Algebra 1/0, which comes back to what I said before. If we had that situation, then Pilots could be replaced by computers, also taking out of the decision making process. But unfortunately (or fortunately) it is not as simple as that:
I had the opportunity to fly into some airport in Indonesia last year. I give you the following scenario, where a immediate Pull Up and go around is not the safest solution:
VMC, Visual Approach, Leaving 1500ft AGL in a 737 turining left base, over hilly terrain, the GPWS comes on, because at the extended base line are more hills. A DC 10 is overhead the airport at 2500 ft about to enter right downwind, a Skyhawk A-4 of the Indonesian Air force is cleared for take off, and a F-28 is approaching the airfield from the north, another B737 from the south. ATC is overloaded.
So in this case, we assest the situation and we found it is safer to land, then to go around and follow the GPWS, because we were VMC and our flightpath lead us around the hills.This particular airfiled requires a left turn towards the airfield on Missed Approach (towards then the other traffic, Right Turn and straight out definately not an option because of Terrain), I think that is when Knowledge, Experience, Training and Common Sense comes into play.
But coming back to what you said, we shouldn't talk about IMC or VMC over the top. Discussion here is out of the question. A GPWS should be followed then immediately when descending into unknown terrain below MSA.