One of the decision you have to make when moving in with a roommate is to make your #apartment smoking or non-smoking. If both of you smoke, it may be a no-brainer to allow smoking inside, but you might want to rethink that decision. If only one of you smokes, the question becomes more complicated. eHow goes over some common roommate rules concerning smoking.
It's probably best for everyone concerned to just confine smoking to outside your apartment. Smoking can excerbate allergies and infuses the smell of smoke into all the furniture, carpets and even the walls, not to mention clothing. These odors can be hard to get rid of when you move out and your landlord may charge you for it through your security deposit. It's also dangerous; if someone doesn't properly dispose of a lit cigarette, it can start a fire. So the easiest course of action is to just smoke outside, even if your roomie assures you they are okay with it inside.
If all the people living in the apartment do decide to allow smoking inside, check your lease to make sure it isn't prohibited and that you know all rules regarding it. Be extra careful that all smoking materials are completely extinguished when finished and never smoke in bed. Make sure all guests know the rules and follow them as well.
Lastly, be open minded about revising your policy down the road. If one of you quits smoking, for instance, it's unlikely they will want to continue to allow smoking inside. If you are the one who quits, be kind to your roommate who is still a smoker, and if you are the one who still smokes, be respectful when you request a change of the rules. The change may lead to a parting of ways, but try to be kind to each other until your lease is up.