I've written an article series that demonstrates how to use WCF to communicate between a task bar (tray icon) app and a Windows Service application.
As an example, the series includes sample code that monitors file change events and sends popup tray app notifications to registered task bar apps.
Here are the first two articles of the four part series.
Tray Notify Using Windows Communication Foundation - Part I
Tray Notify Using Windows Communication Foundation - Part II
Below is an example of the notification window that appears when the user makes a change to any of the files in my documents.
Any comments or suggestions are welcome.