Well, nicotine is an anorectic (reduces appetite), and calorie restriction is the most proven life-extension technique known (in rats), so there is the idea that nicotine --> unintentional calorie restriction --> potentially longer lifespan. Nicotine is a lot easier on the body than other anorectics, which sounds like a weird claim but "other anorectics" means nasty things like methamphetamine and fenfluramine; nicotine is a walk in the park by comparison.

The dangers associated with smoking make it an obvious negative, but nicotine itself is not considered carcinogenic.

It remains to be seen if any of those currently experimenting with intentional calorie restriction will achieve similar results to those seen in animal studies; this would be quite impressive if true.

http://en.wikipedia.org/wiki/Calorie_restriction#Primates