Given it's an addiction, I don't think people should be precluded from making a living due to being a smoker.
I think it's reasonable for certain industries to put restrictions in place on smoking if it affects their work, eg childcare.
Personally I hate smoke and smoking, but discriminating against smokers is one step too far.
It seems to be decreasing in the workplace though. I work in law, and apparently in the 70s and 80s people would smoke at their desk. Now, in my office of about 15 people, nobody, to my knowledge smokes during work hours. Not sure about socially though.