OK, thanks for all the good input. I will go with cement backer board for the kitchen floor tile and I think I will forgo screwing them down; and just use the glue (or latex thinset) to secure them in place. Well, I should say the glue and the WEIGHT of the board + tile.
What about for the bathroom floor? Iím kind of leaning toward a membrane product (like Ditra) since it provides waterproofing. Donít know how many spongy (water penetration) bathroom floors Iíve seen recently. A lot.