http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1277837/
this part was particularly interesting.
"Recent work by Roth et al. demonstrates that THC treatment of murine hepatoma cells caused a dose dependent increase in CYP1A1 gene transcription, while at the same time directly inhibiting the enzymatic activity of the gene product [23]. Thus, despite potentially higher levels of polycyclic aromatic hydrocarbons found in cannabis smoke compared to tobacco smoke (dependent on what part of the plant is smoked), the THC present in cannabis smoke should exert a protective effect against pro-carcinogens that require activation. In contrast, nicotine activates some CYP1A1 activities, thus potentially increasing the carcinogenic effects of tobacco smoke [24]."