I recently stumbled upon one of the most exciting (to me, anyway) Web 2.0 endeavors ever to hit the Internet: Plotbot. Plotbot allows two or more users to work on a script collaboratively. It's designed for the screenplay format, but it could be used just as well for collaboration on a new stage play.
Beyond simply adding and editing scenes and other elements to the script, collaborators can comment on their partners' efforts, and the site's project pages allow for blog postings and document sharing. The project pages act as a dashboard, providing a great deal of useful information about each of your projects.
Plotbot's project page
The system's script-writing engine (powered by AJAX) streamlines the formatting of dialogue, action and sluglines. Here is the "Add an Element" tool: Add an Element to Your Script
And here is how a full scene page looks: Scene Page
As you can see, it's a very slick interface, and every line that is modified has information in the right margin about who made the modification and when. You can also roll a change back if you're unhappy with it.
One other cool and useful aspect -- Each of your projects gets its own RSS feed, so your collaborators can subscribe and get instant updates to their RSS readers when changes are made.
Best of all, the site is completely free. I'd urge anyone doing collaborative script writing to register and check it out.