Well, I'll "weigh in" on this discussion (sorry, couldn't resist).
My daughter is 4 1/2 months old and I have exclusively breastfed using fenugreek, blessed thistle and dom.
My weight has shot up about 20 lbs. I blame myself a little bit...I liked the idea that breastfeeding required an extra 500 calories or so a day. However, I quickly realized that this was not true in my case.
I do eat a big bowl of oatmeal each morning. I know that I can maintain my weight on a no sugar/no wheat/no corn/no corn syrup diet (basically low carb with lots of vegetables). I'm sure the oatmeal makes me dump a lot of insulin to cover it and that may be helping me gain weight.
I did read a comment on another board about a breastfeeding mother who quit the dom and immediately dropped about 16 lbs.
So, yes, I think the dom has contributed to my weight gain. But I haven't been as careful as I should about diet and exercise.
May I say how thrilled I am that I have a full milk supply for my gorgeous growing baby girl though? It was so worth it to see how healthy she is. We love our nursing relationship. I'll miss it when it is over, and hopefully shed the pounds then.