Hmmmmm.... I have not read all the other replies, but this is one thing I've alweays felt very strongly about --- and I don't have kids, but I'm already planning for when the step kids have children!

I told hubby that I want THEIR kids to decide what they want to call me. If its mom-mom, then so beit. (but personally, I always hated it, much for the reasons you love it so much -- my step father's parents were mom-mom and pop-pop... and well, they were not the nicest to my brother and I but were the fdream grandparents of their biological grandchildren) My step kids call their grandfather Fav (or Favie) because thats what they heard when hubby called his dad "father" (my FIL is from the old school that feels children shold respect their parents and call them Father ) ANYHOOO.... they called their grandfather Fav, their Granmom was Nan. Their step-grandmother was Mama. All the names were chosen by the oldest child (my son) and I think its more personal and sweet that way.