This is a first for me! (Score 1)

My roomate smokes, and a few times I've had to open up her PC to change a card or whatever. I've never noticed anything...unusual, just the usual muck that gets collected in there.

I suppose it depends greatly on the location of the computer, and where the smoke is being exhaled. In a still room, its likely that even the fan within the PC will cause a mini-vaccum drawing in larger amounts of air. The real cause of cigarette smell, though, is the placement, and cleansing, of the ash tray. I've lived with a smoker for two years, and unless she just finished, it never smells like smoke. Part of that is because she always vents the room, but a lot of it has to do with the fact that she always keeps her ashtrays clean. You want to avoid a smoke filled room? Clean the ashtrays, that's where a majority of the smell originates.