St. Paul is a troubling individual, but here's the nitty-gritty (from Galatians):
8 But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned! 9As we have already said, so now I say again: If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned!
The word gospel to me here means "revelation", and as such, it invalidates both Islam, which preaches that Christ was not divine and Mormonism, which purports to preach a gospel of a risen Christ in the New World. The Galatians quote is especially damning for Joseph Smith--all but mentioning Angel Moroni by name.
But it is interesting to consider Mormonism a middle-eastern Religion. It is full of Jewish mystical symbolism as well as Freemasonry and Illuminati symbolism. As two of the most rapidly growing religions in the world, what may occur at some far flung distant time when they lock horns with each other?