OK, here we go again. I can't tell you how many times I've read the misconception that not consuming caffeine is part of the Word of Wisdom in the Mormon faith. In the Doctrine and Covenants, section 89 verse 5, it states that "hot drinks are not for the body or belly." The president of our church has clearly stated that hot drinks are coffee and tea. He has also clearly stated that caffeine is not the reason we are to refrain from these drinks.
Let's get real. If Mormons weren't allowed to consume caffeine, then we wouldn't be able to eat chocolate, and I don't know many Mormons who don't do that. So let's get this misconception straightened out. Mormons can, and do, consume caffeine; whether they refrain from drinking caffeinated soft drinks is strictly a personal preference.