I've been working on this one for a couple of days, and am completly
at my wits end.  Is there some method for executing a function or
event in the main thread, when called from a 'worker' thread?  I
understand how it works in the Windows forms world, but what about an
application with no GUI?

I provided example code and the output below.  What I'm looking for is
to have the prog_ShowPopUpEvent method executed by the main thread.

Thanks!

Output:
MAIN THREAD : Sleeping for 1 second
MAIN THREAD : Sleeping for 1 second
MAIN THREAD : Sleeping for 1 second
MAIN THREAD : Sleeping for 1 second
MAIN THREAD : Sleeping for 1 second
TIMER THREAD : timer elapsed
TIMER THREAD : fire show pop up event
TIMER THREAD : show pop up event fired
Main thread exiting.

Code:
using System;
using System.Threading;

namespace ConsoleApplication1
{
   public class Program
   {
       private bool _running = false;
       private event EventHandler ShowPopUpEvent;

       [STAThread]
       public static void Main()
       {
          Program prog = new Program();
          prog.Run();
       }

       private void Run()
       {
           this.ShowPopUpEvent += new
EventHandler(prog_ShowPopUpEvent);
           Thread.CurrentThread.Name = "MAIN THREAD";

           System.Timers.Timer checksTimer = new
System.Timers.Timer();

           //Elapse every 5 seconds
           checksTimer.Interval = 5000;
           checksTimer.Elapsed +=new
System.Timers.ElapsedEventHandler(checksTimer_Elapsed);
           checksTimer.Enabled = true;

           this._running = true;

           do
           {
               Console.WriteLine("{0} : Sleeping for 1 second",
Thread.CurrentThread.Name);
               Thread.Sleep(1000);
           }
           while (this._running);
           Console.WriteLine("Main thread exiting.");
       }

       private void prog_ShowPopUpEvent(object sender, EventArgs e)
       {
           Console.WriteLine("{0} : show pop up event fired",
Thread.CurrentThread.Name);
           this._running = false;
       }

       void checksTimer_Elapsed(object sender,
System.Timers.ElapsedEventArgs e)
       {
           if (Thread.CurrentThread.Name == null)
               Thread.CurrentThread.Name = "TIMER THREAD";

           Console.WriteLine("{0} : timer elapsed",
Thread.CurrentThread.Name);

           //Perform a bunch of checks
           if (true == false)
           {
           }
           else
           {
               //Show the popup window
               OnShowPopup();
           }
       }

       private void OnShowPopup()
       {
           if (ShowPopUpEvent != null)
           {
               Console.WriteLine("{0} : fire show pop up event",
Thread.CurrentThread.Name);
               ShowPopUpEvent(this, new EventArgs());
           }
       }
   }
}