Here's an interesting article on proposals here in the UK: http://www.guardian.co.uk/film/2011/sep/20/smoking-films-automatic-18-rating
The general idea, recommended by the British Medical Journal, is that any film containing smoking should have an automatic 18 certificate. That's roughly equivalent to an R rating in the States, I think. This in theory would help to prevent young people from thinking that smoking is cool, and therefore dissuade them from smoking.
What do people think about this idea?
Personally I'm entirely against it on principle - legislating like this against depicting certain types of behaviour is a slippery slope and is likely to have a 'chilling effect': rather than risk an 18-cert, movies will simply remove smoking. A period film set in the early 20th century would no longer have authentic smoking - suddenly everybody would be cigarette-free, spoiling the authenticity.
Of course, we already censor and restrict based on other aspects - drugs, violence, sex - and I have problems with a lot of that as well. But extending it to things that are bad for you but otherwise morally neutral is a baaaad idea.