This explains a lot, if it's true; posted on alt.coffee by one CmdrJoe.
From the book Buzz: The Science and Lore of Alcohol and Caffeine by Stephen Braun, 1996 (pg. 121) (ISBN 0-14-026845-6):
By a still imperfectly understood mechanism, cigarette smoking "revs up" the liver's caffeine-destroying enzymatic machinery (Benowitz et al. 1989). As a result, the half-life of caffeine among smokers is reduced to an average of three hours (Parsons and Neims 1978). This double-speed elimination of caffeine may explain the long-standing observation that smokers drink more coffee than nonsmokers. Smokers may simply be adjusting their caffeine intake to maintain the same degree of stimulation achieved by nonsmokers.
There's a little more in the alt.coffee message (that link will expire shortly).