Objectivists understand morality as "a code of values to guide man’s choices and actions -- the choices and actions that determine the purpose and the course of his life. Ethics, as a science, deals with discovering and defining such a code." ("The Objectivist Ethics" by Rand)
The morality of an act depends critically on how the person comes to choose it. So considering the morality of smoking amounts to considering: does smoking entail evasion of what the smoker can and should be attentive to? Is the smoker being actively irrational, closing his eyes to something important in his life, negligently breaking loyalty to his hierarchy of values?
Well, it obviously depends on what they are up to, and what they know -- or should know. What appears to be pretty well established at this point scientifically (and thus what smokers should know) is that prolonged and/or heavy smoking bears significant responsibility for serious health effects in most people, adversely affecting lives both in length and quality. While there appear to be (far too many) false and inflated claims about the ill effects of tobacco smoke, that serious smoking dramatically increases the odds of nasty illnesses like lung cancer and emphysema is well-established.
So the bottom line from my perspective is that smoking per-se is neither moral nor immoral, and that it depends on the form it is taking: For example, occasional "social smoking" does not appear to be any obvious risk. And even choosing regular exposure to "secondhand smoke" could be quite irritating and/or disgusting, but it likewise does not appear to pose any obvious health risk. However, prolonged or heavy smoking ordinarily appears to entail evasion of well-established, significant threats to the smoker's values for the sake of their short-term pleasure. I think this last would constitute immorality.
(And because people are often unclear about the proper role of government and the relationship between morality and rights, I must hasten to add: Of course it is the smoker's right to lead his life as he chooses. Laws interfering in "capitalist acts among consenting adults" concerning the production and trade of smoking products -- or laws forbidding a person to smoke in their own space or to permit others to do so -- would themselves be deeply immoral institutionalized rights violations.)