If you're moving your ASP.NET WCF services from IIS 6 to IIS 7, you might not have the SVC handlers added and the 404.3 error might show up:
HTTP Error 404.3 - Not Found
The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.
This fix is to simply need to register the service models with IIS 7. To do this, you have to execute "ServiceModelReg.exe -i" from within %windir%\Microsoft.NET\Framework64\v3.0\Windows Communication Foundation (leave out the 64 if you're running a 32-bit version of Windows)