Request.ServerVariables("LOGON_USER") Returns Empty String in My Project

In an ASP.NET 3.5 project built with VB.NET and run in Visual Studio 2008, the following snippet of code pulls my login name:

Dim EmployeeLogin As String = Request.ServerVariables("LOGON_USER")

In my ASP.NET 4.0 project built with C# and run in Visual Studio 2010, the similar snippet of code pulls an empty string:

string logonUser = Request.ServerVariables["LOGON_USER"];

Both projects call for:

<system.web>
  <authentication mode="Windows" />
</system.web>

Both are being run on my PC.

Why does the VB version work and the C# version not work ...or is this a VS2008 verses VS2010 issue?

~Joe