Interesting questions GR.
For me, we're already past the line in the sand. Not so far past it that I'm going to try to incite a rebellion or anything, but enough that I excercise my legal rights to protest. I write letters to my congresswomen, and I vote and support a third party, participate in their rallies, etc.
For America as a whole I think the point will likely come when it usually does in any nation; when things get really, really bad. Especially when the economy gets really really bad. What happens then is anyone's guess. Could be a peaceful trend back towards Constitutionalism, could be a revolt, could be civil war. Hard to say.
Of course, we could also go the other way, towards a more centralist state, but I think that Americans would fight sooner than see that.
A major breach of Constitutional freedoms would probably trigger a backlash as well. It would have to be drastic, and not done too slowly for people to realize what is going on.