I was at a wedding this weekend and saw a family friend who's a sociologist. We were talking about some of these issues and she mentioned that this question has actually been studied (I must admit that I find it strange to think about people studying us). So here's a link to the study: The Best Time to Have a Baby: Institutional Resources and Family Strategies Among Early Career Sociologists. It is, as the title suggests, not really studying us if us means women in computer science, but it still seems relevant. The study was done by the American Sociological Association and the short version of the summary is:
Those women who give birth or adopt during graduate school have significantly lower odds of obtaining what we have defined as early career success right out of graduate school, even if they obtain institutional resources and use them in career strategies. Their chances improve several years later if they change jobs, when other positive factors are present. Those women who have children after graduate school decrease their odds of obtaining a tenure-track position at a research or doctoral university if they did not obtain this kind of position right after the PhD. Women who continue to delay childbirth do better at obtaining success early in their careers, and, as we have seen, significantly more of them delay childbirth than do their male colleagues.
My short version answer to this question stays the same as before I read the summary (I admit - that's all I've read so far): Have kids when you're ready. That was the advice of someone at the work/life balance panel, and it seemed like good advice to me.