Joseph Smith said that there are so many denominations because of the Great Apostasy and a lack of authority in the church. I disagree. Jay and I are having an interesting conversation about it over on his blog, Mormons Talk. Check it out.
http://mormonstalk.wordpress.com/2007/04/27/why-are-there-so-many-christian-religions/
LDS & Evangelical Conversations
A discussion of differences and similarities between Mormonism and Evangelical Christianity
LDS & Evangelical Conversations 