No caffeine. Still Mormon.
You'll get as many answers as Mormons you talk to about it, but in that realm coffee and tea specifically are what we are to abstain from, not caffeine per se. As caffeine is not called out in the letter of the law, whether it violates the spirit of the law is left to personal interpretation.
If I have a bad night with the baby or otherwise get too little sleep, I won't hesitate to down a 20 oz. Coke when I get to work, but I specifically avoid making that a daily or otherwise too-frequent thing as a caffeine addiction violates my personal interpretation of the spirit of the law.
Or in other words, I'm sure ol' Mitt would be fine having a Coke with Ben.