In this post, we’ll explore how you can leverage the cool Taskbar functionality of dynamic overlay icons and multi-state progress bars. A central Windows 7 tenet’s that the "User Is in Control"; Windows 7 “removed” the System Tray Icon area. By default, almost all the tray icons’re concealed. Consequently, it’s safe to assume that large number of the notification balloons’ll also not be visible and most users’ll not see them. You can read more about the updates to the Notification Area here. To compensate for this lack of notification, Windows 7 Taskbar offers Overlay Icons and Progress Bars. By using overlay icons and progress bars, your application can provide contextual status information to the user in spite of the lack of a System Tray Icon area and even if the application’s window does not display. The user doesn’t even’ve to look at the thumbnail or the live preview of your app – the Taskbar button itself can reveal whether you have any interesting status updates.
Full Article: Windows Blog