Wow that is pretty cool! I created a file called helloworld.cs

// helloworld.cs - Gtk# Tutorial example
 
using Gtk;
using GtkSharp;
using System;
// using System.Drawing; (the guide had this but must be for windows
// did not work on linux)
 
public class HelloWorld {
	// This is a callback function. The data arguments are ignored
	// in this example. More on callbacks below.
	static void hello (object obj, EventArgs args)
	{
		Console.WriteLine("Hello World");
		Application.Quit ();
	}
 
	static void delete_event (object obj, DeleteEventArgs args)
	{
		// If you return FALSE in the "delete_event" signal handler,
		// GTK will emit the "destroy" signal. Returning TRUE means
		// you don't want the window to be destroyed.
		// This is useful for popping up 'are you sure you want to quit?'
		// type dialogs.
 
		    Console.WriteLine ("delete event occurred\n");
		    Application.Quit ();
	}
 
	public static void Main(string[] args)
	{
		// This is called in all GTK applications. Arguments are parsed
		// from the command line and are returned to the application. */
		Application.Init ();
 
		// create a new window
		Window window = new Window ("helloworld");
 
		// When the window is given the "delete_event" signal (this is given
		// by the window manager, usually by the "close" option, or on the
		// titlebar), we ask it to call the delete_event () function
		// as defined above. The data passed to the callback
		// function is NULL and is ignored in the callback function.
 
		window.DeleteEvent += new DeleteEventHandler (delete_event);
    
		// Sets the border width of the window.
		window.BorderWidth = 10;
    
		// Creates a new button with the label "Hello World".
		Button btn = new Button ("Hello World");
    
		// When the button receives the "clicked" signal, it will call the
		// function hello() passing it null as its argument.  The hello()
		// function is defined above.
		btn.Clicked += new EventHandler (hello);
 
		// This packs the button into the window (a gtk container).
		window.Add (btn);
 
		// The final step is to display this newly created 
		window.ShowAll ();
 
		// All GTK applications must call the main loop: Application.Run
		// Events are processed and dispatched here.
 
		Application.Run ();
	}
}

To compile with Mono, type

david@tux ~$ mcs -pkg:gtk-sharp-2.0 helloworld.cs

This results in a file helloworld.exe, that you can execute with mono:

$ mono helloworld.exe

And a window pops up with helloworld!!! inside it

To see what devolopers are doing with mono and C# visit Muine