With some trepidation I have posted a draft of my paper "A Priori Contributions to Scientific Knowledge". The basic claim is that two kinds of a priori entitlement are needed to ground scientific knowledge. I find one kind, called "formal", in the conditions on concept possession, and so here I largely follow Peacocke. For the other kind, called "material", I draw on Friedman's work on the relative a priori.
Even those not interested in the a priori might gain something from the brief case study of the Crowe et. al. paper "A Direct Empirical Proof of the Existence of Dark Matter". What is intriguing to me about this case is that the "proof" works for a wide variety of "constitutive frameworks" or "scientific paradigms" in addition to the general theory of relativity. I would suggest that this undermines the claim that such frameworks are responsible for the meaning of the empirical claims, such as "Dark matter exists", but I would still grant them a role in the confirmation of the claims.
Update (July 10, 2009): I have removed this paper for substantial revisions.