I do believe the notion that nicotine will boost your metabolism to such a degree that you will lose wait is only half wright. That metabolism boost may help in the short term, but how long will it last? If someone has smoked for a decade for example, how likely is it that nicotine is still boosting their metabolism and helping keep them thin? Not very. The body adapts after a time, after all.
Perhaps the idea that nicotine speeds up metabolism for as long as someone uses it comes from the wait gain brought on by changes in metabolism when someone gives up tobacco. That nicotine might not keep you think after a time, but the sudden absence of it could cause you to gain weight.