Nicotine demethylates when heated into nornicotine. This reacts with saliva to form the carcinogenic N-nitrosonornicotine -- which is also found in any variety of cured tobacco.

While not itself carcinogenic, it is incredibly hard to separate nicotine from cancer, unfortunately.

http://en.wikipedia.org/wiki/Kim_Janda#Nornicotine

http://en.wikipedia.org/wiki/N-Nitrosonornicotine