It should be pretty obvious by now that smoking cigarettes is bad for your health. What's not as obvious is that it might be bad for your Mac, too. According to The Consumerist, two different people got turned down for AppleCare maintenance because their Macs were used in a house with a smoker. Both people appealed their cases all the way up to Steve Jobs, and both of them lost.
I've opened up a few computers that spent time in a house full of heavy smokers, and the insides weren't pretty. There was this disgusting brown resin built up all over everything, and it pretty much smelled like an ashtray stuffed full of 5-year-old cigarette butts. Though it's probably a stretch to call this "a biohazard" like in one of the cases The Consumerist cites, AppleCare agreements are worded loosely enough in their limitations of coverage that Apple seems perfectly within its rights to deny coverage in these two cases:
The Plan does not cover:
Damage to the Covered Equipment caused by accident, abuse, neglect, misuse (including faulty installation, repair, or maintenance by anyone other than Apple or an Apple Authorized Service Provider), unauthorized modification, extreme environment (including extreme temperature or humidity), extreme physical or electrical stress or interference, fluctuation or surges of electrical power, lightning, static electricity, fire, acts of God or other external causes.
It's that "other external causes" clause that's the catch-all, although you could probably argue that an atmosphere filled with smoke counts as an "extreme environment" as well.
Bottom line: if you have to smoke, you might want to step away from the computer first. With the investment that your Mac represents sitting there on the desk, why take the risk of messing it up in an easily preventable way?