OK I'm going to get drawn in to this debate now (I was avoiding it).
Tamum is right that the effects of smoking and breastfeeding (whether together or separately) cannot be isolated. But equally the causes of all the things mentioned (cot death, brain development etc) cannot be isolated and this thread has reminded me quite how much I dislike the attitude that says you should not do anything which has been statistically connected in any way with harm to your baby.
No parent does everything they possibly can to minimise risk to their children. Household accidents are the biggest killer of children: so do we all live in bungalows with padded furniture? No. Because parenthood is not about doing everything by the book. It's about finding a balance.
Furthermore, I don't see why everyone compares everything to smoking, as if being a smoker was far and away the highest risk of all. It isn't - if Spanish statistics are anything to go by, then having a swiming pool is.
I certainly don't mean to denigrate anyone with a swimming pool (I want them to be my friends ). In fact, speaking of Spanish statistics, it is possible that even the link between smoking and cot death has been overstated. I would love to see a study on this, but as far as I know, cd rates in Spain are lower than in the UK, despite the prevalence of smoking being higher and the accepted advice being to put babies to sleep on their bellies. Other factors must be playing a part: perhaps diet or climate, or perhaps the fact that smokers here come from all classes - they are not concentrated in lower income brackets as they are in the UK. Equally on another thread someone mentioned that formula-fed babies in Europe don't seem to be as prone to gastro-enteritis as in the UK.
You may have guessed that I smoke and breastfeed, by the way. I do not smoke near either child unless outside, but I do smoke. It's not relevant but I smoke because when I gave up I completely failed to remove the mindset which says that smoking=stress relief - I know consciousl that it doesn't, but I personally deal with stress better when I smoke. I don't feel guilty about it really - I will give up some time soonish - but I do resent my habits being used as an example of something that is terrible to do as a mother.
I'm not looking for a fight (if I was, I'd copy and paste bits of this to an MMR thread), but as others have said, please don't feel guilty if you didn't breastfeed or didn't do everything that people told you to. And please don't be smug if you did.
I feel better now.