Pres. Hinckley stated on his Larry King appearance that we abstain from caffeine. From the transcript:

"CALLER: I was wondering about some of the guidelines in dietary restrictions Mormons live by, and how strictly members follow it. Because I was reading, once, the word of wisdom. My impression was that its major point was that one should respect all life, including animals and, as such, only consume them when absolutely necessary to sustain life, and to then eat them sparingly. But I've noticed that Mormon -- this is rarely followed by Mormons, and I'm wondering if this has anything to do with, as reported by "TIME" magazine.
LK: OK, president?
GBH: Oh, I don't know. You've read a part of the word of wisdom. The word of wisdom covers many things. It covers the excessive use of meat, as I see it. It covers, in a very particular way, the use of tobacco and alcohol.
LK: By saying no?
GBH: By saying, by proscribing those things.
LK: No to caffeine?
GBH: No to caffeine, coffee and tea.
LK: Do you know why? Dietary -- in the Old Testament were based on the health of animals and stuff.
GBH: Well, the wonderful thing is that the Book of Mormon -- I mean, the word of wisdom has shown to be fruitful in what -- the accomplishments."