Impersonation and NetworkCredential

I need to pass a NetworkCredential object with the credentials of the currently impersonated user to a web service from an asp.net application.

My code looks like this:

WindowsIdentity windowsIdentity = HttpContext.Current.User.Identity as WindowsIdentity;
WindowsImpersonationContext context = windowsIdentity.Impers开发者_JS百科onate();
try {
    var client = GetClient();
    client.ClientCredentials.Windows.ClientCredential = CredentialCache.DefaultNetworkCredentials;
    Log("WindowsIdentity = {0}", windowsIdentity.Name);
    Log("DefaultNetworkCredentials = {0}", CredentialCache.DefaultNetworkCredentials.UserName);
    client.DoSomething();
} finally {
    context.Undo();
}

I had understood that CredentialCache.DefaultNetworkCredentials should give the credentials of the currently impersonated user, but it is not the case.

The log messages I get are

WindowsIdentity = TESTDOMAIN\TESTUSER
DefaultNetworkCredentials = 

Am I doing something wrong? If so, how do you get a NetworkCredential object for the currently impersonated user?

A somewhat lengthy article in MSDN explaining the options to obtain network credentials in ASP:

How To: Use Impersonation and Delegation in ASP.NET 2.0

Another blog article on the topic (though I didn't check whether the solution actually works:

.NET (C#) Impersonation with Network Credentials

It's not possible to use the asp.net impersonated user (Current.User.Identity) for network authentication, it only works locally.