Interesting thread! I'm leaning towards a little more than a 3 year gap for several reasons. First is nutritional - I don't feel quite ready yet, body-wise. I need to be stronger, physically, too. DD's birth was really traumatic (ketosis in labour, massive hemorrhage post-partum, post-partum anemia, low milk supply, PPD, etc....) and I feel like the pregnancy took more out of me than I realized. I was all happy about not gaining much weight, but I think I should have made more of an effort to gain more. I ate well, but very little, because I am tiny and all my "height" is in my legs, my stomach got squished very early in my pregnancy to the size of a gerbil and I just couldn't eat very much. So, fetuses being the greedy little goobers they are, DD sucked quite a bit out of me. (Also, she was nearly 9 lbs when she was born. Damn DH's genes...but she is also gloriously healthy.)
Also, DH is currently looking for a tenure-track position but we have no idea where that will be, and there are places in the world (cough half the USA cough) where I would NOT want to give birth. So we definitely want to wait until we get that sorted out. But on the flip side of things, I'm 35, and nearing the end of my suitable reproductive era. So if we end up waiting more than another 3 years, I think DD will be an only child, or we will adopt.
And, FWIW - caedmyn I agree with you on the nursing while pregnant thing. That's a no-go for me too. DD is starting to get close to weaning - in fact today was the first day EVER she didn't want a booby first thing on waking up, and she's down to nursing just then and to go to sleep - but I don't want to rush her, I'd like it to be as much of a mutual decision as possible. But, having seen one mama I know deal with toddler nursing style and pregnancy boobs, and another mama I know deal with the trauma and squabbles of a 3-year-old sibling jealous of the booby time a newborn gets, there's no way I would nurse through a pregnancy.