Everybody is cement-board happy. I don't like using it on
floors because it has no tensile strength. Use cementboard
on walls.
As a professional I use hardibacker on floors due
to it's tougher tensile strength and compression strength,
and it doesn't expand and contract. It is also highly
water resistant.

A good alternative for the average DIY homeowner would
be exterior-grade (CDX) plywood. A 1/2" layer glued and
screwed over a 3/4" existing floor would give the needed
1 1/4" subfloor to receive tile (use exterior grade screws-
NOT drywall screws).
Exterior-grade plywood can be gone over with multi-purpose
thinset* and is the "correct" type of plywood to use for tiling.
Look on the back of any brand or type of multi-purpose
(polymer modified) thinset, and you'll see "exterior grade
plywood" listed as an acceptable surface for it's use.
It is also very resistant to moisture. It is what roofers use
(in Florida anyway) under roofing. It is also inexpensive,
easy to cut and easy for a homeowner to work with.

NOTE: this would only be for interior use, where the
temperature and humidity is kept at a relative constant.
Plywood will expand and contract in temp. extremes.
*Always use the highest quality "flex-set" thinset over
plywood.

thetiler