Steve Ballmer announced VSTO support for Outlook add-ins in his Tech-Ed keynote today.  This is exciting news--VSTO Outlook add-ins solve all the problems people have encountered while trying to build managed COM add-ins for Outlook using IDTExtensibility2.  For example:

1) VSTO Outlook add-ins load into their own AppDomain--there is no longer a need for you to create a custom C++ shim.
2) VSTO Outlook add-ins solve the "Outlook won't shut down" issue--you no longer have to track every Outlook object you use and set it to null and force a garbage collection or even worse call ReleaseCOMObject on anything.  Your add-in will always cleanly shut down.
3) VSTO Outlook add-ins solve the "Trust all installed templates and add-ins" issue.  Because the VSTO Outlook add-in loader technology uses the same runtime and security model used by VSTO, if your add-in is trusted by .NET policy, it will work even when Trust all installed templates and add-ins is unchecked.
4) The VSTO Outlook add-in project is all wired up and ready to go.  Create a new project, press F5, and Outlook starts up.  No extra settings to configure.
5) VSTO Outlook add-ins have a nice strongly typed programming model that is very similar to the VSTO Word & Excel programming models.  Say goodbye to the old COM-centric IDTExtensibility2 interface.  The Outlook add-in project has a code item called ThisApplication.cs or ThisApplication.vb where you write your code.  Instead of being passed a weakly typed application object, the ThisApplication class you write your code derives from a base class that wraps the Outlook application object so you can use "this" in C# and "me" in VB to get to Outlook Application properties, methods, and events.  Here what code in a VSTO Outlook add-in looks like:

using System
using System.Windows.Forms;
using Microsoft.VisualStudio.Tools.Applications.Runtime;
using Outlook = Microsoft.Office.Interop.Outlook;
 
namespace OutlookAddin1
{
public partial class ThisApplication
{
    private void ThisApplication_Startup(object sender, System.EventArgs e)
    {
        MessageBox.Show(String.Format(
          "There are {0} inspectors and {1} explorers open.",
          this.Inspectors.Count,
          this.Explorers.Count));
      this.NewMail += new Outlook.ApplicationEvents_11_NewMailEventHandler(
        ThisApplication_NewMail);
    }



void ThisApplication_NewMail()
    {
      MessageBox.Show("New mail!");
    }

private void ThisApplication_Shutdown(object sender, System.EventArgs e)
    {
      MessageBox.Show("Shutting down.");
   
}

#region VSTO generated code
    private void InternalStartup()
    {
      this.Startup += new System.EventHandler(ThisApplication_Startup);
      this.Shutdown += new System.EventHandler(ThisApplication_Shutdown);
    }
    #endregion
  }
}

Download VSTO support for Outlook now!   http://download.microsoft.com/download/1/4/4/144ef6a4-920b-4aed-9ed8-ed4a7ba7193a/Setup.exe

This installer installs VSTO support for Outlook on top existing VSTO Beta 2 .