The Book of Mormon musical speaks to the rising social and economic influence of Mormons and the attempt by The Church of Jesus Christ of Latter Day Saints to go mainstream (as well as some ribbing that comes as a result).  For example, two Mormons, Mitt Romney and John Huntsman are running for the Republican presidential nomination in 2012.   

I have a great love and interest in Mormonism (as apparently do Stone and Parker).  For those who want to learn more, the PBS Documentary on the History of Mormonism is mind blowing.  

Below is one of the cleaner songs from the musical (All American Prophet about Joseph Smith).  The musical constantly makes reference to other musicals...this one is in the style of Andrew Lloyd Weber (Jesus Christ Superstar, Joseph and the Amazing Technicolor Dreamcoat).  Their understanding of Mormon history and theology is quite good as you'll hear.

Money line:  "You mean the Bible is a trilogy?"  

Update:  For those who are more drawn to the hardcore side of Southpark, there is this song.  Warning:  It is quite graphic, brilliantly awesome, insanely politically incorrect, seriously disturbing, and so over the top.  Don't say I didn't warn you.