Charles Spitzer wrote:
the above not withstanding, as long as it's dry, plywood doesn't
warp,
especially if securely fastened to the studs.

the answer is yes, you can. use mastic instead of thinset.

But it does give. If you stand on a piece of securely attached plywood
and bounce up and down, it may move up and down with you. Over time,
this kind of motion can lead to cracks, if not in the mastic, then
certainly in the grout.

On the wall it probably doesn't matter, but I would (and did) use
backerboard on the floor over plywood. I wouldn't want to gamble with
mastic over plywood.

--
Jennifer