Subjective conscience and objective moral truth

On the topic of conscience and authority, Steve is correct to emphasize the Christian tradition's deep respect for the subjective dimension of conscience.  This does not mean, nor do I read Steve as suggesting, that there is no objective moral truth.  Since the time of the Scholastics (and as foreshadowed as far back as Paul's epistles), Christianity has employed a dual framework for understanding conscience: synderesis is a person's general, non-deliberative moral knowledge, and conscientia refers to the freely chosen application of that knowledge to a particular set of facts.  The moral truth to which synderesis pertains does not change simply because we do not accurately perceive its substance or practical import.  At the same time, it is wrong to act against one's conscience, even an improperly formed conscience.  The gap between moral truth and application is not necessarily fixed; it may be closed through experience or teaching.

What then do we do with a conscience that is not, in the Church's understanding, accurately perceiving and/or applying moral truth, especially if the gap cannot be closed through experience or teaching?  At some point, is a Catholic supposed to defy the claims of her own conscience and submit to Church teaching that runs contrary to her moral convictions? 

Take birth control, for example.  Suppose that a Catholic reads all of the relevant Church teaching on the subject, talks to her priest, prays, etc., but that she still remains convinced that the use of contraception within marriage is not immoral.  At this point, how should the conversation go between the woman and her Church?  The Church can say, "we respect your conscience, but it is improperly formed, so in acknowledgment of that fact, you should submit to Church teaching."  The woman responds, "how is it improperly formed?"  There is no response that the Church can make on this point that has not already been considered and rejected by the woman.  Ultimately, it is a straightforward claim of authority by the Church.  If the woman's only reason for concluding that her conscience has been improperly formed is the Church's assertion that it has been improperly formed, that seems to create tension with: 1) a fulsome respect for conscience; and 2) the belief that knowledge of the moral law, as pertains to contraception at least, has been written on our hearts.

I can see how this problem can be worked out from the standpoint of ecclesiology (as members of a faith tradition that is bigger than ourselves, not all of our truth claims require our individual assent), but I have a harder time working it out from the standpoint of conscience (for which individual assent is central).  Obviously, I have more questions than answers on this tension, and I welcome others' views.