PAH's are what everyone keeps jumping up and down about.

Yes Cannabis smoke does contain more of these agents than tobacco smoke (no really it does!) but it isn't as simple as that....

From here :: http://www.harmreductionjournal.com/content/2/1/21

Thus, despite potentially higher levels of polycyclic aromatic hydrocarbons (PAH) 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