While nicotine may not be as carcinogenic as other chemicals in cigarettes, I would not call it "not harmful at all."

First of all, nicotine is a fairly powerful poison. One to two drops of pure nicotine under the tongue can kill a healthy adult. Nicotine derivatives are also toxic to insects, and are often used as pesticides (Which is most likely what the tobacco plant uses them for).

Secondly, a recent study has shown a link between long term nicotine therapy (gum or patch) and lung cancer. It seems that under acidic conditions in the body, nicotine can turn into NNK, a chemical known to cause lung cancer. While it should not disuade anyone from using nicotine replacement to stop smoking, it adds another reason to kick the habit.

Information about the link between nicotine and cancer from: http://www.smh.com.au/news/0012/02/national/national17.html