The .NET Compact Framework 3.5 is only a subset of the full framework. One of the limitations is that you cannot talk to WCF services using ServiceContracts.
Microsoft released a long time ago PowerToys that can generate a WCF proxy to overcome this problem.
One of the application of the PowerToys is NETCFSvcUtil.exe. This tool generates two files. One file does the magic, the other is the implementation of the proxy client. You can add these two files to your Windows Mobile solution in order to communicate with WCF services. Unfortunately it only supports the basicHttpBinding and not the more powerfull wsHttpbinding.
I added the path of the PowerToys to my Visual Studio 2008 Command Prompt variables. I added the PowerToys path (C:\Program Files\Microsoft.NET\SDK\CompactFramework\v3.5\bin) to the vsvar32.bat file which is located in C:\Program Files\Microsoft Visual Studio 9.0\Common7\Tools.
Now it looks like this:
@set PATH=C:\Program Files\Microsoft.NET\SDK\CompactFramework\v3.5\bin;C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE;C:\Program Files\Microsoft Visual Studio 9.0\VC\BIN;C:\Program Files\Microsoft Visual Studio 9.0\Common7\Tools;C:\WINDOWS\Microsoft.NET\Framework\v3.5;C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727;C:\Program Files\Microsoft Visual Studio 9.0\VC\VCPackages;%PATH%