One of the many new WCF features in Visual Studio 2008 SP1 is a mechanism to control whether the built-in WCF test host application (WcfSvcHost.exe) should automatically launch when debugging other projects in a solution. Prior to this release, there was no way to control this without disabling WcfSvcHost.exe by removing the special ProjectTypeGuid that controls this feature from the .csproj file.
Now, you can simply right click on your WCF Service Library and select Properties. Then in the project properties area you should find a tab called WCF Options, which looks like this:
Simply uncheck the box to disable the WcfSvcHost.exe from starting when debugging other projects within your solution (such as your own custom host application). However, if you attempt debug the WCF Service Library directly (as the startup project), the WcfSvcHost.exe will still launch in that case.