ETA: Wednesday, June 13.
Soon Website Pro will come with another helpful developer tool—a Staging Environment.
A staging site gives you a place to test design changes, plugins, themes, and WordPress updates before publishing them to your clients' live site. Then, once you and your clients are happy with how they look, you can push them to production with the click of a button.
To access the staging environment, head into Website Pro, then find the Staging tab on the top navigation.
New Staging Environment