Even if we decided to ignore the evidence that the dangers from second hand smoke are totally overblown, distorted, and fabricated (read: they don't really exist), and proclaim that environmental tobacco smoke is a great threat that we need to protect non-smokers from (

), it should be fairly obvious to libertarians which situations where smoking would be permissible:
The Commerce Clause be damned, the desires of the property owner are paramount. Even in a world of no smoking regulation, a property owner could still decide that he does not want smokers on his property and thus forbid them from doing so. The government has no say in the matter.
In a perfectly libertarian society, even the roads that some people prefer that smokers do not smoke while driving on would be privately owned. The owner would determine what behavior would be acceptable while driving on his roads (now, how private ownership of infrastructure would be organized is another whole talk entirely).
If you're not an anarcho-capitalist and you think that in a libertarian society that there should still be a government which has control over roads and the behaviors permitted and regulations enforced on them, then you might have a case if you want to use violence against those who choose to smoke while driving on public roads.
If you don't like smokers on sidewalks or in front of buildings, then so what? Avoid them. It is not like we are constantly accosted by smokers everywhere we go. It is not a grand smoker's conspiracy to blow smoke in the face of those who do not like the habit (it is quite the contrary). The fact that a majority of people do not like tobacco smoke only provides a social incentive for people not to smoke or stop smoking. Let normal social pressures persuade people to voluntarily kick the habit. Smokers already suffer from the social stigma to a point that it can seriously inhibit career progress and important networking (as people would prefer not to befriend smokers if they find it repulsive, as many people increasingly do). Hooray for market forces at work in the social realm.
For the record, I am a loyal Nat Sherman cigarette smoker and I understand that many people do not like smoking, so I am courteous about my behavior and do not litter or smoke at the entrances of buildings, and I always ask permission if I may smoke in someone's private residence or even outside at a social gathering. Nothing to do with government or regulation.
After all, it is totally understandable why people do not like tobacco smoke. It makes sense that one would be as courteous as possible if he so desired to remain in good standing with others. Those who are not suffer the social consequences (along with the health risks, obviously). It is the same with alcohol. It is obviously within people's right to get drunk at a bar, but it probably isn't beneficial to your social standing to make a fool of yourself.