Smoking progressively blocks up your arteries and veins, and starves your body and brain of oxygen, which reduces the amount of concentration you can exert.
Also, if you're having a withdrawal pang from a cigarette it severely detracts from your concentration. I imagine that if you attempt to quit, and then have a poker session and suffer a few bad beats. Your body will trigger that "need to light up" feeling, and if you remind yourself that you quit and that you can't smoke anymore, all those thoughts of smoking can make you stop focusing on the game!
It's the small things that contribute greatly to your play, I think.
Though I've never really had problem with smoking and poker... but I can remember times when I was simply unable to effectively study for a test or do something else that required mental effort, while I was attempting to quit.
So yeah, I think the worst place to be is in the transition from smoker to non-smoker. So, if smoking helps you calm down and not tilt after some bad beats, I think you should realize that it's not anything in the cigarette that's helping you, it's just a feeling of relief from withdrawal caused by not smoking for the past X minutes or Y hours. Since I've quit, I've found far better ways of dealing with these sorts of negative emotions, like tilt 
