Short Answer: Yes.

Long Answer:
Marijuana smoke has been shown to contain many of the same chemicals that are in tobacco smoke, including many of the same polycyclic aromatic hydrocarbons and tars. Unlike tobacco smoke, marijuana smoke cannot cause emphysema because it does not obstruct small airways like tobacco smoke.
There has yet to be a reported case of lung cancer as a result of smoking marijuana alone and several studies suggest that lung function is not signficantly impaired in cannabis smokers.
What convinces me most that marijuana smoke is 'safer' than tobacco smoke is the fact that marijuana smoke isn't radioactive. What I mean by this is cigarette smoke is radioactive. Radio-fucking-active. Now im no doctor, but I know well enough that anything radioactive can't be very good for you. Especially if you're inhaling it.
Among the radioactive constituents of tobacco smoke is Polonium 210. This is the only agent of tobacco smoke that has been shown to cause cancer when inhaled in experiments involving cute little lab animals.
http://www.ukcia.org/research/cancer2.htm
http://nepenthes.lycaeum.org/Drugs/T...ancer.rad.html
http://www.mapinc.org/drugnews/v03/n1432/a02.html
http://my.webmd.com/content/article/23/1728_57309

This is not to say that marijuana smoke is completely innocent of negatively impacting the health of your lungs. I think it can be said definatively however that it is a hell of a lot safer to inhale smoke that doesn't contain any sort of radioactive components.