How do you retrieve a list of logged-in/connected users in .NET?

Another option, if you don't want to deal with the P/Invokes yourself, would be to use the Cassia library:

using System;
using System.Security.Principal;
using Cassia;

namespace CassiaSample
{
    public static class Program
    {
        public static void Main(string[] args)
        {
            ITerminalServicesManager manager = new TerminalServicesManager();
            using (ITerminalServer server = manager.GetRemoteServer("your-server-name"))
            {
                server.Open();
                foreach (ITerminalServicesSession session in server.GetSessions())
                {
                    NTAccount account = session.UserAccount;
                    if (account != null)
                    {
                        Console.WriteLine(account);
                    }
                }
            }
        }
    }
}

Ok, one solution to my own question.

You can use WMI to retreive a list of running processes. You can also look at the owners of these processes. If you look at the owners of "explorer.exe" (and remove the duplicates) you should end up with a list of logged in users.

Here's my take on the issue:

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace EnumerateRDUsers
{
  class Program
  {
    [DllImport("wtsapi32.dll")]
    static extern IntPtr WTSOpenServer([MarshalAs(UnmanagedType.LPStr)] string pServerName);

    [DllImport("wtsapi32.dll")]
    static extern void WTSCloseServer(IntPtr hServer);

    [DllImport("wtsapi32.dll")]
    static extern Int32 WTSEnumerateSessions(
        IntPtr hServer,
        [MarshalAs(UnmanagedType.U4)] Int32 Reserved,
        [MarshalAs(UnmanagedType.U4)] Int32 Version,
        ref IntPtr ppSessionInfo,
        [MarshalAs(UnmanagedType.U4)] ref Int32 pCount);

    [DllImport("wtsapi32.dll")]
    static extern void WTSFreeMemory(IntPtr pMemory);

    [DllImport("wtsapi32.dll")]
    static extern bool WTSQuerySessionInformation(
        IntPtr hServer, int sessionId, WTS_INFO_CLASS wtsInfoClass, out IntPtr ppBuffer, out uint pBytesReturned);

    [StructLayout(LayoutKind.Sequential)]
    private struct WTS_SESSION_INFO
    {
      public Int32 SessionID;

      [MarshalAs(UnmanagedType.LPStr)]
      public string pWinStationName;

      public WTS_CONNECTSTATE_CLASS State;
    }

    public enum WTS_INFO_CLASS
    {
      WTSInitialProgram,
      WTSApplicationName,
      WTSWorkingDirectory,
      WTSOEMId,
      WTSSessionId,
      WTSUserName,
      WTSWinStationName,
      WTSDomainName,
      WTSConnectState,
      WTSClientBuildNumber,
      WTSClientName,
      WTSClientDirectory,
      WTSClientProductId,
      WTSClientHardwareId,
      WTSClientAddress,
      WTSClientDisplay,
      WTSClientProtocolType
    }

    public enum WTS_CONNECTSTATE_CLASS
    {
      WTSActive,
      WTSConnected,
      WTSConnectQuery,
      WTSShadow,
      WTSDisconnected,
      WTSIdle,
      WTSListen,
      WTSReset,
      WTSDown,
      WTSInit
    }

    static void Main(string[] args)
    {
      ListUsers(Environment.MachineName);
    }

    public static void ListUsers(string serverName)
    {
      IntPtr serverHandle = IntPtr.Zero;
      List<string> resultList = new List<string>();
      serverHandle = WTSOpenServer(serverName);

      try
      {
        IntPtr sessionInfoPtr = IntPtr.Zero;
        IntPtr userPtr = IntPtr.Zero;
        IntPtr domainPtr = IntPtr.Zero;
        Int32 sessionCount = 0;
        Int32 retVal = WTSEnumerateSessions(serverHandle, 0, 1, ref sessionInfoPtr, ref sessionCount);
        Int32 dataSize = Marshal.SizeOf(typeof(WTS_SESSION_INFO));
        IntPtr currentSession = sessionInfoPtr;
        uint bytes = 0;

        if (retVal != 0)
        {
          for (int i = 0; i < sessionCount; i++)
          {
            WTS_SESSION_INFO si = (WTS_SESSION_INFO)Marshal.PtrToStructure((System.IntPtr)currentSession, typeof(WTS_SESSION_INFO));
            currentSession += dataSize;

            WTSQuerySessionInformation(serverHandle, si.SessionID, WTS_INFO_CLASS.WTSUserName, out userPtr, out bytes);
            WTSQuerySessionInformation(serverHandle, si.SessionID, WTS_INFO_CLASS.WTSDomainName, out domainPtr, out bytes);

            Console.WriteLine("Domain and User: " + Marshal.PtrToStringAnsi(domainPtr) + "\\" + Marshal.PtrToStringAnsi(userPtr));

            WTSFreeMemory(userPtr); 
            WTSFreeMemory(domainPtr);
          }

          WTSFreeMemory(sessionInfoPtr);
        }
      }
      finally
      {
        WTSCloseServer(serverHandle);
      }

    }

  }
}