This report just popped up today. If you smoke in front of your Mac and you take it in for AppleCare service, you will be denied based on OSHA rules according to Apple. The tar and gunk inside is considered "hazardous material", and Apple employees are not allowed to work on a device in that condition.
Computer fans suck in air to run across components and heat sinks to cool internal components. Smoking around any computer will also suck in smoke inside your computer. All that tar and nasty stuff binds to the internal fans and components making them sticky, and all the dust that gets sucked in will become permanently adhered inside your computer.
Apple has denied AppleCare claims to two people that brought in computers in this condition, although apparently smoking damage is not currently outlined in the AppleCare warranty. I would expect this to change shortly, adding smoking as a voidable warranty condition.
If you smoke, take it outside.
Erik