I've just got my copy of this collection through from Amazon, and skipped straight to chapter 4 on the a priori. I was interested to notice Michael Devitt (pp. 107-8) discussing one of the questions I asked in my most commented post so far, namely why we should assume that experience only tells us what is the case, rather than what must be the case. (Devitt, unsurprisingly, is in favour of a holistic, web-of-belief account of how it can.)
Even more interestingly, BonJour (p. 100) argues that the way to solve a certain kind of regress problem ('the application of a propositional insight concerning the cogency of ... an inference would require either a further inference of the very sort in question or one equally fundamental') is to claim that a priori rational insight cannot be always (perhaps cannot be ever) 'propositional in form'. Instead, an a priori insight must (often) be 'a direct grasping of the way in which the conclusion is related to the premises and validly flows from them'. This raises several questions, including the question of how best to make sense of how grasping 'the way in which ...' could amount to something other than the grasping of a proposition, or how, even if it is different, it could amount to grasping something which does not itself (by BonJour's - internalist - lights) stand in need of justification like a proposition does. I always suspected the solution to this kind of problem was to reject the (famously regress-generating) internalist premise.
Another interesting question the suggestion raises (which as far as I can tell BonJour does not address) is whether, if non-propositional a priori insight is possible, there could also be empirical grounding for these non-propositional things, whatever they are. It's hard to see an obvious reason why there could not, though I think if there could that would undermine this argument of BonJour's for the existence of a priori insight.