According to this article, Apple refuse to repair anything that has been used by a smoker because nicotine is listed as a hazardous substance. They claim it is a biohazard and therefore the warranty is void because the techs will get second hand smoke.
Whilst I understand that working on a smokers PC is usually pretty nasty and that the tar does get anywhere and can damage things, I don't see it being an actual biohazard or being able to give you second hand smoke. Put some gloves on and away you go...