It is not generally against any laws to discriminate against smokers. There may be a state or two that protects smokers so check to see if your state does. If it does not, then yes, not only can you use the question but you can discriminated based upon the answer. However, you would need to make sure it doesn't lead to disparate impact on a protected group (for example all the smoker applicants are also all over 40).
Is it best business practice to do so? Not in most cases. But if the top mgmt wants to use it as a hiring criteria, they can. They can also use it as a factor in health insurance premiums (by charging higher premiums for smokers or giving a break on premiums to nonsmokers).
More than one or two states have these laws, so you better check.
Go to http://slati.lungusa.org/states.asp, click on the state, and then on Smoker Protection Law.
I think it might be interesting to see someone make an ADA claim based on their nicotine addiction. It's not illegal. It's a recognized physical addiction. Would you feel comfortable asking them if they are an alcoholic or, more extremely, if they have diabetes and, if so, if they take good care of themselves so that it won't be a problem for the company? All exageration aside, I know some companies ask and I think I've read something about some states allowing differential health insurance premiums based on smoking status.