For Whom Does the Church Speak?

I just noticed this snippet from the Roberts confirmation hearing:

Mr. Specter, a Pennsylvania Republican, wanted to know if Judge Roberts agreed with what Senator John F. Kennedy told a group of Protestant ministers in 1960: "I do not speak for the church on public matters, and the church does not speak for me."

"I agree with that, senator, yes," said Judge Roberts . . . .

I don't necessarily disagree with Judge Roberts, but this raises a question for me: when the Church speaks on public matters, for whom is it speaking?  I understand that the Church does not impose, but simply proposes; however, it still must be proposing views that are deemed claims of truth from someone's perspective.  So on public matters, is the Church's perspective somehow separable from the laity's perspective, or at least potentially separable -- i.e., can and should faithful Catholics discern for themselves whether they will embrace the Church's stated perspective?  Or is Roberts implicitly defining the category of "public matters" to involve issues where prudential judgment is key, and where the Church may not have more expertise than the laity?  Or does the Church speak for the laity in their roles as citizens, but not when they take on public roles like President (JFK) or judge (Roberts)?  What exactly does it mean for Roberts to say that the Church does not speak for him?

Rob