I don't think there's any conclusive evidence that smoking causes hairloss, but I did read an article that linked it to premature graying. Gray hair is, for the most part, drier and more brittle than your regular hair, and given that smoke dries out your hair as well, I'd think it could lead to breakage, though that's just my theory.
Either way, it's definitely not good for your hair, I mean look at a lot of long time smokers that have that fried-out dry hair.
I just quit almost three weeks ago, so it's too soon to tell if it's made much difference, but I can say that my hair smells a lot better. I'm hoping that with time it'll go back to the softer, shinier look it had before I started smoking (though I think that male pattern baldness had a part in that change as well).