I just saw it at Blockbuster as well. It’s a drama (fiction, no less) depicting the early Mormons’ divine revelation telling them to travel across part of Eurasia, build a boat, and sail to America to establish Jesus’s first-string of followers around 600 B.C.
Mormons claim that their ancestors came to America around that time, and had great cities and wars with the bad people (that became the current American Indians), and of course had the true faith. Granted, the claims that the livestock they had here in America to live off didn’t actually come from Europe until after 1300 A.D. is of little consequence, and the fact that these “great cities” have been disproven archaelogically, this Book of Mormon story is still staunchly defended. I’m sure the movie is just trying to provide some legitimacy to their claim and to get their foot in the door with Hollywood, since Mel Gibson seems to have the last word to date.