Everyone seems to be quoting Brooks and making efficiency arguments, none of which are particularly wrong, mind you, but I think they've missed a rather essential point. Programming is, generally speaking, a lot more art than science. Oh sure, you can get a CS degree and learn a heck a lot about algorithms and solving "typical problems" at a good school and thus increase your knowledge of the science side substantially, but the only CS grads I've ever seen go on to truly become Software Engineers (vs taking detours into marketing, sales or non-engineering management) were the ones who truly loved it and considered 40 hours a week to be insufficient, if anything, because they loved to create things and writing software was a particularly satisfying way of doing just that. I'm not saying that all software engineers need to be artists at heart, but the very best ones are, and no employer will ever say "I'm looking for the 2nd or 3rd best over here! Keep those resumes coming, please!"
By saying you only want to work part-time, you're essentially telling a prospective employer that you're just not that into it in comparison to the other candidates who are devoting every waking hour to software and contributing their work to various open source projects when no better outlet for their creativity is available. If an analogy will help, imagine your next restaurant meal: Who do you want to make it? Somebody who doesn't really care about cooking and is just slapping your meal together or a chef who really loves cooking?