To the real estate editor: I don't know if it will help with old grout stains, but I had to tell you about X-14 Instant Mildew Stain Remover, which I found through a neighbor.
Living in such a humid climate, I have had ample experience with X-14 -- all of it good -- and no scrubbing.
One word of caution: It will bleach out clothing. Also, I do not use it around children or pets. This poses no problem, because it dries rather rapidly and then is harmless. X-14 is made by White Laboratories Inc., PO Box 15335, Orlando, FL 32858. Lyndee Congleton Huntsville, Ala.