http://www.lds.org/ensign/2008/12/the-energy-drink-epidemic?lang=eng
But the author says to be careful to avoid becoming addicted. To borrow a phrase from the WofW, caffeine should "be used with prudence and thanksgiving."
Some will point out Pres. Hinckley's 60 Minutes interview, in which he agreed with Mike Wallace's statement that Mormons don't drink caffeinated sodas.
http://www.youtube.com/watch?v=pLFDP_MeYhg
I'm okay with following that (since I rarely drink sodas, anyway), but I will say this: I'm an Exec Sec, and my Sunday "shifts" at the wardhouse usually are 7 hours long. To make it through sleep-inducing EQ meetings, SS lessons, and sac. mtg. talks, I don't hesitate to take 100 mg of caffeine (either via a 100 mg Jet-Alert pill or by popping half of one of the 200 mg caffeine pills available under various labels in every supermarket and gas station in America).