Could the title be updated to indicate whether we are talking about smoking, or other means of ingestion? Smoke inhalation was already known to be bad, regardless of what the smoke was made from.
With the 60 or so cannabinoids (http://medicalmarijuana.procon.org/view.answers.php?question...) and 50 or so hydrocarbons there seems like a lot of scope for causing health issues. Especially given cannabis smoke is more carcinogenic than tobacco smoke.
None of that necessarily provides a reason to ban marijuana but it may give people reason to switch to safer modes of consumption, or taking of only specific active compounds.