This is an unfortunately names topic since we seem to be concentrating on Web GUIs wheras I think most people are thinking Swing/AWT/Windows GUI at first sight.
Nonetheless, I have been tasked with providing a project framework to allow collaborative work between server-side Java developers and web content creators and graphics artists. Like everyone here has learned, this is tough. And, like everyone, we have created a process that works for us.
First and foremost, we have automated as much of the building and deploying of the application as possible. Our testers work with an internal version of the site that is recreated twice a day. The build and site deployment is capable of being performed on a user's workstation or in a server farm with a couple of command-line arguments.
As for the web content, we spent a lot of our design time creating a system with which content can develop with little or no knowledge of the back end. Using standardized XML responses or JSP taglibs, the content writers create and prototype without ever really caring what the developers are doing. They simply put code or placemarkers where results should be returned. Getting dynamic information is no different than writing image tags. This has worked very well for us so far.
Now, if we could only incorporate XP into the developer side of things, I'd be very happy. I think we can since: we're under 15 people, are pretty much paired off already, have little in the way of process other than coding standards and configuration management standards.
Jesse