Problem:'

More specifically, is nicotine in the concentrations that smokers receive when smoking cigarettes toxic? I know that in great enough concentrations it can be toxic (but then, so can just about anything else, including oxygen) and I know that in plants it is used as a defense against insects and can even be used as an insecticide. However, it has always been my understanding that nicotine is irrelevant as far as the harmful effects of smoking go.
I recently had a conversation with another biologist who had just quit smoking and had done quite a bit of research on the subject. He said that nicotine itself is in fact bad for you and, therefore, that tobacco-less alternatives to cigarettes (such as electronic cigarettes) are still harmful because of the nicotine alone.

Does anyone have any more information on this?

Perhaps some references?

Or, even better, a detailed explanation of the pathways involved?

Again, I stress, not about nicotine's toxicity in general but about its harmful effects on vertebrates (preferably human) at the kinds of concentrations one could expect to ingest when smoking. Please justify it.