http://dx.doi.org/10.1038/nm.4274
Edit: Please read abandonliberty's comment below before reading mine. I didn't properly elucidate the position on the harms v. benefits tradeoff (net harm) the way abandonliberty did.
My own opinion: it wouldn't be terribly surprising to see many harmful substances only be harmful to a majority subset of a population space, though that's why the scheduling system (theoretically) exists -- to allow drugs to be classified based on harm v. benefit to various populations. Whether Nicotine should be scheduled is... well, given the number of people addicted to it, probably not the easiest conversation to have. I originally thought it was Schedule II (high abuse potential but with some medical benefit, which would certainly fit our potential future understanding of nicotine based on studies like this), but it turns out my original hunch was faulty googling.