Like villa and neal said - use backer board (over plywood). I just did my kitchen floor, and it turned out beautifully ... I took mine down to the subfloor. The reason you need backer (concrete) board is that a wood floor will flex, eventually cracking your tile. The backer board won't flex like that. I suggest that you trowel a thin layer of thinset on your floor, then screw down your backer board (using screws specifically made for that purpose). The thinset under the backer will fill any (minor) imperfections in your floor to help prevent flex. I'm just a DIY-er, SO, any of you pros disagree with my advice, or have a better way?