Newsweek & the Washington Post have a joint blog 'On Faith', which explores the Mormonism as mainstream question in light of the PBS Frontline documentary. How does one define mainstream?
I recall Dr. Ed Harrell lecturing on Pentecostalism, his academic specialty, and discussing the reaction he received in grad school decades ago when he began studying it. His fellow students wondered why he would go to a Pentecostal tent revival rather than a 'mainstream' Episcopal service. Even then the handwriting was on the wall about the Episcopal Church.
Count the number of Mormons in the USA and the world, and compare it other religions. Of course, from a historical perspective, Mormonism is not 'mainstream' but rather created from whole cloth by Joseph Smith's imaginings. Still, can we say that when our Senate majority leader and a major Republican Presidential candidate are both Mormon that it is outside the mainstream of American life?