I still say that the problem with the floor in question could likely be totally unrelated to the cement board issue. There is no way that people can be so confident in diagnosing this particular problem given the small amount of information in the original post. You can quote chapter and verse from whatever organization you want, but in most real world applications the difference between plywood and cement board isn't what makes or breaks the floor. You don't even know how much plywood there is on this floor, how it is attached, the size and condition of the joists etc. etc. I am mostly playing the devil's advocate and saying that it is impossible for everyone to be so preachy and sure of themselves when there is no way you can be sure that you are right. As I said, everything must be done correctly, including backerboard installation, for the grout not to crack. Using backerboard is no panacea. Its just as easy to screw up a floor using cement board. If this bathroom has 2x4 joists, all the backerboard in the world isn't going to help, so the posts I'm questioning might not have been helpful and were jumping to conclusions without adequate information. Maybe the person used unsanded grout, maybe they used too much water, maybe they didn't wait 24 hours for the thinset to dry, oh wait, we don't even know what the person used to install the tiles! How can anybody be so sure that the plywood is to blame if you don't even know if they used thinset, mastic or chewing gum to install? This isn't how I think this board should work. Take me to task b/c I don't prefer backer board, fine, but try to find out more before you go off so sure of yourself.