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golgol-
I know it seems weird but, no never in an upstairs bathroom. I know plenty of people will do it but I like to be able to guarantee my work and I've seen too many fail. Some fail fast but 2-3 years is not uncommon and even 5-10 years would be a bother for me. If you ever get a chance to look at the deflection of some of the victorian floors from below (i.e. when the ceiling is removed in the room below) then you might reach the conclusion I have! Despite the fact that the ceiling will add some rigidity it's still a lot of flex.
Mostly people are following style cues from other countries who have different building methods and climates as well as different age of buildings.
Airhead
Oooohhh, badass :) If I had to guarantee the same (no breakages) to a client over floorboards I would replace them with 18mm ply screwed down at 10cm centres on the joist. It could still move enough to break clay. Without knowing the current floor heights I'm not sure what would work best in terms of thickness but I'd be tempted to level the concrete floor with self levelling and then match the height of it with ply on the joists of the wooden floor (so you could calculate your self levelling level based on 25mm ply for example). Then lay your tiles direct on the self levelling and the ply but have a grout joint at least (or possibly even silicone) where the substrates meet because it's that area which will be most prone to cracking.
A lot depends on the size of the tiles though. Generally I just don't fit tiles on wooden floors, ever.