I've bored for Mumsnet on nicotine addiction in the past and Jimjams post sounds spot on to me (as does www too..)
I was really, really lucky that cigarettes made me sick once I was pregnant, I still had the odd few while I was getting my head around the concept (and still in total denial) but since then zip BUT I know I would fall off the waggon with just one now, even after 5 years (I've even stopped for 6 years in the past and gone back to 40 a day)
I've been on two Mumsnet meets and had to avoid the smokers because I know I'll be back to square one within a week.
Nicotine is an awful drug and if you've been lucky/sensible enough to avoid it, great but I'm afraid you will never understand those of us who have been caught by it. A number of surveys have shown it's easier to come off heroin than nicotine, perhaps partly because cigarettes are more readily available (depending on your circumstances).
Yes, we all know the dangers of smoking, particularly when pregnant but unless you've been there I really don't think you can understand the way it screws your head.
Just look how many people give up during pregnancy only to go back to it afterwards, theoretically they should be well over the addiction, they see the adverts about kids breathing smoke and yet they still smoke, nicotine really does make the brain think of excuses for that behaviour.
Right I've finished my glass of wine so I'm off to bed !