"You've got to ask yourself one question: 'Do I feel lucky?' Well, do ya punk?" If the answer is no, pick up one of these. It will be a few bucks well-spent when you're trying not to spill while drinking out of your Nalgene bottle in your down sleeping bag.
This handy little piece of insurance protects against that block of frozen sports drink crashing against your face as well. It removes easily for chipping away said frozen drink, and has a little tab to assist in that maneuver.