Thank you all so much. I'll probably go with 1/4" or 1/2" CBU, depending on the height the adjacent hallway is going to be. I'd rather not step up into the bathroom if I can help it.
I lurk around here quite a bit and have learned so much. I think I might get enough nerve to try the tub surround as well. Won't happen 'till early next year, so I have plenty of time to do my homework.
Muskymike, the ditra looks like a good product, but I'm kind of limited to shopping at the box stores. I'll keep my eyes open as I do my research, though.
Now I'm off to meet the painter and tape more tyvek seams.
Holly