According to separate reports, Apple denied coverage for two users in 2008 based on the fact that OSHA lists cigarette smoke as a biohazard. Both techs refused service to the computers not because the smoke damaged them, but because it’s considered hazardous to a tech’s health to work on a contaminated unit.
I can attest to how gross the inside of a computer can get after being used in a smoking household, but I’ve never seen anyone deny warranty coverage because of it.