I saw a special flexible cement for wooden floors advertised in Topps Tiles, but I can't comment on its use apart from the fact it is quite dear.
The tiler I use when Im doing bathrooms always wants 18mm plywood screwed into the T&G or whatever at 300mm spacing. He then adds a plasticiser to the mix to give it some flexibility.When I got some "wood" cement he complained it took too long to set so he couldnt grout the same day.
I tiled my bathroom floor onto a wooden surface with no problems. I put 12mm plywood on top of the floorboards and screwed down every 6", I used the adhesive that is specially formulated for wooden floors (states that you can tile directly onto the tongue & groove floorboards)and I have had no problems with it but it does require min 24hrs drying time before you can walk on it, but in saying that I have since seen a 'rapid'version of the same stuff that can be walked on after an hour or so
When I did this I misread the guidelines and put thin hardboard down instead of thick MDF or plywood. I put 300 screws in at appox 10" spacing to try and get some stiffness into the structure. To be honest I don't think I'd could have done the job if I'd had to put ply or similar down as it would have been a lot of work fitting it round a bathroom suite and made the floor an inch higher. I laid the tiles over a few weeks (most in a weekend, the edges after I'd sourced a tile cutter) using a flexible adhesive, meant for tiling onto wood. Half are now grouted. One tile, which I think I fixed badly has come lose. The rest seem fine. The grout hasn't cracked (You can use the flexible adhsive as grout but I wanted a different color) despite being normal cement based, so I'm happy. It's been done about 3 months so far. I expect I will have problems in the future, but I'll just have to wait and see.
6mm, (minimum) plywood ontop of floorboards and fix tiles with a two part flexible adhesive, not cheap but worth it. I use this method all the time when fitting underfloor heating and never have any bother. PS dont forget the flexible grout!