I agree with all said, except use Durock anyhere direct exposure to water is even a possibility. In a tub/shower area, that means the ceiling too. The only pain is that using Durock, one MUST use thinset mortar for the joint compound to make a waterproof barrier.
This is part of the difference between a 5 year and a 25 year bathroom.
Also, look into a product called "DITRA". It is a waterproffing system for wet areas and is worth the money.
http://www.schluter.com/6_1_ditra.aspx