I believe 'talc' is out from a safety perspective for babies, but what you buy as baby powder these days is often actually 'corn starch' instead. Although my J&J powder in my purse is actually talc, I believe they do a cornstarch version as well.

My friend who uses cloth diapers never uses anything, but as I understand it, plastic coated diapers (Huggies, Pampers etc) can cause issues if you change the baby without thorough cleansing and drying first (which one imagines you cannot do in a airplane loo). The powder absorbs the extra moisture from the skin, and prevents nappy rash. As someone highly allergic to nappy / diaper rash cream (sudocreme, zincofax etc etc) corn starch is certainly my preference!