Well a friend of mine quit smoking and gained 20 lbs but I think a lot of that is because in a desperate effort to keep herself busy at night, she started baking.
Every day she'd tell me she made muffins or baked cookies or pie or something ... and she'd take a lot of it to work (I"m sure her coworkers either loved or hated her!) but there was still the tasting during the cooking and the snacking after ... and so forth.
My dad quit smoking and dropped weight, but when he quit smoking he also started WALKING the golf course again instead of riding the cart, because he could do it w/out coughing up a lung. So you know ... cause and effect there too.
I have read different studies about nicotine depressing the appetite, of course it gives you a metabolic boost due to the stimulants, and I've heard a bit about it affecting how you absorb nutrients ... but I'm not completely up to date on all that research.
Most of what I've seen with friends and family is more about habit building and oral fixations than anything else.