I only use treated wood for permanent pens (I use cheapest I can find for temp setups) and haven't had problems. There is a new variety of treated wood available that is not supposed to leak toxins, and has no arsenic (the toxin of concern in treated woods). The new type is more expensive by a bit, but supposedly lasts just as well.
Anyway, I will continue using normal treated wood as long as I am doing work outside. I do not seal or paint the wood either. I use it only for walls, though, which may be better than using it for floors, where the turtle would be in direct contact with it at all times.