While working on the "MCE Add-In Launcher" add-in I ran across a good post from Aaron Stebner over on the Media Center Sandbox Blog that discusses how to use a named mutex to prevent you from launching multiple instances of your own plug-in.
This is good practice but many add-ins don't do it, its particularly important to me now because of the "MCE Add-In Launcher Add-In", you see the MCE SDK doesn't give add-in authors the ability to enumerate the running add-ins and mark which one is active; all you can do is launch another instance of the add-in.
That means your dependent on the add-in making sure only one is ever running, this is good from a design standpoint but I wish they gave us access to enumerate the back stack and the ability to move items to the top if you are not the owner of the page although as a security guy I see the problems with this.