Extract from Wikipedia's entry for "pink":
"In Western culture, the practice of assigning pink to an individual gender began in the 1920s. From then until the 1940s,
pink was considered appropriate for boys because being related to red it was the more masculine and decided color, while
blue was considered appropriate for girls because it was the more delicate and dainty color, or related to the Virgin Mary. Since the 1940s, the societal norm was inverted; pink became considered appropriate for girls and blue appropriate for boys, a practice that has continued into the 21st century."
http://en.wikipedia.org/wiki/Pink