During the recent Public Sector Developer Conference in Washington, DC, I was asked about tools support, beyond the class designer, for modeling WCF services. My answer was to use the Service Factory Modeling Edition for Visual Studio 2008 from the patterns & practices team. Here's a quick screenshot of what the design surface looks like:
You will also want to check out the community site at http://www.codeplex.com/servicefactory. The community site contains hands-on labs for both using and extending the Service Factory, discussions with customers and field, known issues, and roadmap information. In the near future the site will include presentations, demonstration videos, and community contributions.