Thanks to Valerie for this Frugal Baby Tip!
I didn't believe that sun-bleaching was that effective until I tried it, but it really is! At least here, in a semi-desert at a high elevation, it takes less than a half-hour sitting in the sun to whiten them all up. My diapers have all gotten sullied and stained over the long winter, but the days are getting warmer and one day soon I'll set them all out again and have whitewhitewhite diapers again. Not that the stains bother me greatly, but knowing that I can get rid of them, I do like to do that at times.
Give sun-bleaching a try before taking more chemical-oriented (stain removers) or labour-intensive (rinsing) measures.