I think this topic is (or should be) extremely important to everyone who believes in self-defense as well as those who choose to carry any kind of knife on a regular basis. My guess is that most people at this forum fall into both groups. Here are some of my thoughts, your mileage may vary:
1) Many states define weapons in terms of intent, so (theoretically) if it is not a weapon in your mind it is not a weapon at all until you decide otherwise. On the other hand, if we are honest with ourselves, when we talk about "defensive use" we are talking about use as a weapon and under an intent-based law we would be carrying a weapon. Absent actual use as a weapon, I would think that cops, DAs, etc. are going to be guided by the appearance and demeanor of both you and your knife.
2) Always remember that one permissible response to the weapons question, at least in the US, is some polite variation on "I decline to answer your questions at this time." Under my understanding of the law, this constitutionally protected response should not by itself give an officer cause to search you or your vehicle. That being said, my understanding of the law and $2 will get you a cup of coffee at any courthouse in the country.
3) I have seen opinions on this forum (one I believe involved the Spyderco SPOT) discussig how the marketing of a particular knife could play a role in an LE decision to charge or confiscate a knife. ("Really, officer, AFCK stands for Advanced Folding Craft Knife!"). It seems like a responsive manufactuer could address this concern by offering a "craftsman" version of various models that are marketed to emphasize the tool aspects of a knife.
Great topic, thanks for hearing me out.