Today, Slate's Explainer runs down the history of gender-neutral clothing. Slate has previously featured slideshows explaining the history of gender-specific colors. I have also written up some guidelines for telling boys from girls in 19th-century photos.
At the end of the Slate article, Brian Palmer notes that, "Despite an uninterrupted quarter-century of lacy dresses and flowered pink headbands, observers of high-end children's clothing designers detect a trend back toward gender-neutral clothes." I wish he had provided a citation for this, because I haven't seen any evidence yet. Maybe the places I've been looking have not been high-end enough, but, as someone who has purchased approximately a million articles of infant clothing in the past few months, gender-neutral (or even just not overly frilly) clothing is hard to find. My quest for tiny corduroy overalls without appliqued butterflies or trucks on them continues largely without success.