Over at All Too Common, there is a discussion about a growing trend in the Presbyterian Church in America (the conservative split off of the mainline PCUSA) to reckon Roman Catholic baptisms as invalid on account of the PCA position that Roman priests are not duly ordained ministers of the Gospel.
This is interesting. I can remember a PCA minister once telling me that he thought "Catholics should be rebaptized." I thought this was merely the view of one ignorant minister. But apparently not.
The real crux of the position is that it necessarily "unchurches" John Calvin himself. John Calvin, of course, received a Roman Catholic baptism. By these new standards, it seems that Calvin wasn't really a member of the visible church, because he never received a "valid" Reformed baptism. This seems to be the logical conclusion.
The Catholic position is that any Trinitarian baptism is valid, regardless of who administers it. There must only be an implicit desire to "do what the Church does" when administering baptism. See Catechism of the Catholic Church #1256.