In order to get it working for WCF, you need to provide a custom error handler by implementing the IErrorHandler interface:

public class ElmahErrorHandler : IErrorHandler
{
    public void ProvideFault(Exception error, MessageVersion version, 
                             ref Message fault)
    {
        var dummyRequest = 
            new SimpleWorkerRequest("dummy", "", new StringWriter());
        var context = new HttpContext(dummyRequest);
        
        var elmahLogger = Elmah.ErrorLog.GetDefault(context);
        elmahLogger.Log(new Elmah.Error(error));    
    }

    public Boolean HandleError(Exception error)
    {
        SDExceptionHandler.DoHandle(error);
        return true;
    }
}

This error handler needs to be added to the stack of error handlers. You can do this in a couple of ways, for example by providing a custom attribute that implements IServiceBehavior and then applying this attribute to your service class.

Next you need to add some configuration to your web.config and your all good to go:

<system.web>
    <httpHandlers>
        <add verb="POST,GET,HEAD" 
             path="MyService.Elmah.axd" 
             type="Elmah.ErrorLogPageFactory, Elmah" />
    </httpHandlers>
</system.web>

<elmah>
    <errorLog type="Elmah.XmlFileErrorLog, Elmah" 
              logPath="C:\MyServiceLog\"/>
</elmah>

What’s nice about this approach is that you don’t need to run the WCF service in ASP.NET compatibility mode, which is a major bonus.

Hope this helps