Also, keep in mind there's a difference between corn starch and talcum powder. People tend to use the term "baby powder" as a generic catchall for either. I'd opt for corn starch there over the talc. I don't know if either is biodegradable, but I'd venture that corn starch is better than the talc in that regard
Baby powder is Talc and corn starch.
Corn starch is organic. [25% amylase (enzyme) and 75% amylopectin (polysaccharide and polymer of glucose found in plants) Translation: stored sugar and stuff to break it down VERY biodegradable]
Talc is a mineral. [will degrade into smaller units but not quickly]
Edit: I am in college to teach science, just covered talc, last year I learned about corn starch. Checked my figures on wikipedia.