My first question is, is this really tongue and groove hard wood flooring or simply the old wood under-layment which would also be tongue and groove.

My sister made this mistake. She ripped out all the carpet and linoleum in the house and is simply walking around on the under-layment over her crawlspace. She thinks she has a real find but I doubt it's oak or even a valuable hardwood used for flooring.

That being said, if you want to just put a cement/hardy board over it, I would do everything I could to stabilize the floor, cross (oh heck I forgot the term)pieces between the joists, strengthening the joists as much as possible to really give you a solid foundation.

Personally, the greenovation person in me is screaming, NO, don't waste that wood just to save yourself a little work.