I think smoking would be an awesome thing to implement. The way I see it though, it shouldn't be something dwarves do instead of doing other things. The way it is now, dwarves go to statue gardens, diningrooms or other meeting zones when they have no jobs. Smoking could just be another activity for them do partake in when idle, and would cause happy thoughts. It makes for a really good export, but I can see one problem.
The carrying system as it is now would leave all smoking dwarves practically disabled because of the pipe being carried around in their hands, even when not smoking, or cause them to have to go back and forth to their rooms all the time to pick up smoking gear. One sollution is that dwarves who like smoking would go pick up pipes from finished goods-bins, and carry them around in special pipe-pouches made at the leatherworks or by a clothier. The pouches would work like quivers, and would hold a certain ammount of tobacco and a pipe. Smoking dwarves without pipe-pouches would have to store their pipe in their rooms, causing them to have to go pick them up before having a smoke. One would then designate an area for smoking, like one would designate a meeting area.
Smoking would generate small ammounts of smoke, and there could exist dwarves not fond of smoking. If they were they to breathe in this smoke, this would cause mild negative thoughts, depending on their grade of dislike. Another way to store pipes would be to designate stockpiles in the smoking areas, where pipes were put after usage and picked up before, but I guess this would be a bit unhygenic, even for dwarves. Tobacco could be stored in a stockpile in the smoking area for the dwarves to pick up before smoking, or in its own room to avoid huge tobaccofires caused by an unlucky spark. Tobacco would be grown on farm plots, and harvested and dried.