Chuck is right. Compromise is what it's all about... and it's not so insane for your wife to be more concerned as your son becomes more aware and able to ask questions she might find tougher to answer... like: Why is Daddy doing that?
On the other hand, if the divorce thread is what got you wondering, then perhaps there is a larger issue? I could be reading too much into the post, but if that's where your mind is taking you then that's the problem i would solve for first. If you guys are rock solid, then she'll take you with the cigars. Esp. if it's been ok to this point.
I'd talk it over with her and make sure she is aware of how stopping would upset you just as much (or more) than how smoking upsets her. Once you two agree that you're both caught with skin in the game, it may be easier to find a middle ground.
For my wife, I set up some ground rules for myself and my cigars. Only after the kids are in bed, good hygene, a promise to put down my 'gar when she has something better in mind... wink wink... maybe if you come to the table with some solutions in mind that address her primary objections she'll be more open to collaborating with you?
Anyways... that ended up longer than I intended. Best of luck brother.