When you create a WCF Service in VS 2008, it writes the service config into web.config.
Usually you can right click on web.config in Solution Explorer and select Edit WCF Configuration.
However I have noticed that the context menu item does not always show up.
The answer: Select Tools > Wcf Service Configuration Editor instead; this will bring up the editor with nothing loaded, so you will then need to open the relevant config file. However, just starting the editor seems to be enough to remind Visual Studio about that context menu item, so just exit the editor and go back and right click on web.config - the Edit WCF configuration option has returned... ;-)
Cheers
Ian