It’s discussed in detail in the cite that Kimmy_Gibler gave. According to that author, as far as the RCC is concerned, LDS doctrine proposes three separate deities that act together, rather than a triune deity as in RCC belief. Since (in this view) the LDSs do not believe in the same Holy Trinity the RCC does, their baptisms will not be valid in the RCC.
Slow night in Quebec? 
Ahhh, I get it. The Mormon trinity, while mysterious and confusing and self-contradicting, isn’t nearly as mysterious and confusing and self-contradicting and dizzying as the RCC trinity. Therefore, when Mormons baptize in the name if the father, son, and holy ghost, they don’t really mean it.
Thanks for clearing that up.
Not “they don’t really mean it,” but “they mean something different than we do.”
Seriously though, the issue that they don’t mean it. The issue is that they are baptising in the name of an entirely different entity to which they have given the same name. That actually makes sense, as much as anything can make sense.
To give an imperfect analogy. If someone named their dog “the Flag of the United States”, and they named their cat “the Republic for which it stands”, they could swear allegiance to their pets all they liked, but nobody would accept that they had actually taken the oath. Sure, the words are identical, but the concepts aren’t in any way the same.
In the same way, the LDS trinity isn’t in any sense the same entity as the RC trinity. It has the same name/names, but it apparently doesn’t share anything else in common. The God part presumably isn’t even eternal or the original creator. That’s a pretty clear proof that the entities are not the same.
Do the Roman, Eastern, and all the Protestant churches have identical trinities? Do their differing theologies make them pseudo-Christian too? How about the JWs and Adventists?
Catholics and classical Protestants–Lutherans, Calvinists (Presbyterians), Episcopalians, Methodists, Baptists, etc.–pretty much do agree about the Trinity. The Eastern Orthodox conception of the Trinity is, in theory, slightly different from the Western (Catholic/Protestant) conception–see the Filioque controversy (if you dare).
Jehovah’s Witnesses are Nontrinitarian; they believe Jesus Christ to be the Son of God, but not “God incarnate” or “true God from true God”. In this view, Jesus is a sort of super-archangel and mediator between God and humanity, with a special position distinct from (and higher than) all other creations, but nonetheless separate from God (Jehovah). I believe the Seventh-day Adventists are Trinitarian, the same as other Western Christian denominations, but of course they are quite insistent that worship should be on the seventh day–Saturday–not on the first day–Sunday. I think there are some other doctrinal differences as well.
They aren’t identical, but they are essentially the same: three aspects of the eternal creator god. Thus they are acceptable for RC baptismal purposes. That is in contrast to the LDS trinity, which is three distinct and independent entities cooperating.
The issue isn’t whether they are “psuedo-Christian”. that is a term that only you have used. The issue is whether a Baptism to the LDS Holy Trinity is being attributed to the same entity known to the RC Church as “The Holy Trinity”.
Don’t know much about the Adventists, but JWs don’t even *have *a concept of a Trinity. The JW “Son” is a distinct entity, created by God and wholly separate from him. The JW Holy Spirit is just an expression of activity, and no more distinct from the God then your left elbow is distinct form you. So they certainly don’t qualify as a RC acceptable baptism.
Oh, and as a result of their theology, Jehovah’s Witnesses are also likely to be considered non-Christian or heretical or what-have-you by Trinitarian Christians.
I am quite willing to accept Mormons as fellow Christians.
This does not mean I agree with their crazy doctrines. It does not mean I think their baptism is valid. It does not even mean I think they will go to heaven. It’s simply a recognition that they profess faith in Christ, and by virtue of that fact can be accurately described by the demographic term “Christian”. They may be hellbound heretical Christians, but they are Christians nonetheless.
The Catholic,Orthodox, and Protestant churuches all share a pretty common belief regarding the Trinity. There are small, hair-splitting differences between specific denominations, (look up filioque, for example), but they all look to the Nicene Creed for the definition of the Trinity as both Three and One.
The Seventh Day Adventists were not strictly Trinitarian at their founding, but they embraced more and more Trinitarian thought as they included more Christians from other denominations who brought Trinitarian beliefs with them. They do not spend a lot of effort describing the Trinity and they use language that seems odd to some other Christians, but they now share the same basic Trinitarian beliefs as the majority of Christians, and have for many years.
Jehovah’s Witnesses are explicitly not Trinitarian. They believe in Jehovah as God, with nods to Jesus and the Spirit as agents, (for want of a better word, since I am not that well read on their dogma), of Jehovah.
= = =
As to “pseudo-Christian,” I do not choose to set those sort of labels. If a group wishes to self-identify as Christian, I am not going to leap up and denounce them. In a discussion of particular beliefs, I may note that one group or another does or does not follow the Nicene Creed or, in other contexts, specific passages of Scripture or the early Councils, but I draw no conclusion regarding their legitimacy as Christians. Not my job.
There are several Christian groups in the Middle East, (despite living among Muslims for 1400 years), who broke with the mainstream Christian church at early dates who may also be more Trinitarian or less Trinitarian, but again, I do not choose to rank them as tothe quality of their Christianity.
Thanks for the replies. I hadn’t realized that the major Christian groups all accepted the Creeds. I was raised LDS, and was taught that belief in Christ as savior, and belief in the Bible (especially KJV), were what makes one a Christian. To a Mormon, all the competing Christian groups are hell-bound heretics, but they’re all Christians.
Regardless, as an ex-Mormon I will always consider myself to be ex-Christian.
I wouldn’t call that wishy-washy. The comment presupposes that Mormons are **not **Christians: “Christians respect…” I suppose we’ll have to save the issue of what historical research confirms that the Bible is the inspired Word of God for another time. 
I didn’t see your question when this thread was new, Polycarp, so forgive me for answering you two years later. The bishop I was referring to was the Right Reverend Carolyn Tanner Irish, 10th Bishop of Utah (now retired). According to her brief biography on Wikipedia:
I agree with you that a conditional re-baptism would have been appropriate.
I thought that we were Telestial-bound heretics and that the Outer Darkness/Lake of Fire was only for the truly wicked & apostates.
The concept of Hell is a little unclear in Mormonism. The word appears a lot in LDS scripture. It can refer to “Spirit Prison” (a temporary purgatory where disembodied spirits are given one last chance to convert), “Telestial Kingdom” (the lowest level of heaven, which will be somewhat nicer than mortal life), and “Outer Darkness” (an eternal ennui populated by Satan, 1/3 of the host of heaven, and apostates like me). All references to fire and brimstone are regarded as figurative.