Pope John Paul II says they aren't, but having seen many Catholics on Mormon threads, it seems that he and Catholic teachings don't carry much weight among pro-Mormon Catholics, who first loudly announce that they are Catholic, and then proceed to fight like tigers for Mormonism as a Christian church.
Like I said before, this is a response to the post-death baptisms carried out by the Mormon church. If any Mormon baptism is held to be valid then all those spurious (in the eyes of the Church) post-death baptisms risk the implication of validity. This is an inter-church spat.
One of these days (or knowing how this goes, one of these centuries) this may get changed.
It does not get to the crux of my point - It does not matter what Mormon doctrine is; we tolerate other politicians, leaders, business partners, and etc., of various faiths in spite of extreme differences and our often bloody histories. Mormons should be no exception.