Julien,

Right, so if infallibility is defined in terms of truth entailment and truth entailment is defined in terms of necessity (a warrant W entails a proposition P iff W necessarily implies P's truth), then I think you're right that all necessary (and possibly contingent) truths will be infallibly justified. So my proposal is to define infallibility in terms of the W for P rendering the probability of P 1. So yes, this would imply that a fallible proposition is one whose warrant gives it a probability of less than 1. This perhaps would require revising the initial definition of infallibility as involving warrant that is truth entailing and falsity precluding (although I need to think about this).