Plugin Code ONLY on Post Edit Page

Using @One Trick Pony’s method, this should do what you want:

global $current_screen;  // Makes the $current_screen object available
if ($current_screen && $current_screen->base == "edit") {
    // Edit-page/post-only code here    
}

If you want it on the add OR edit screens, then:

global $current_screen;  // Makes the $current_screen object available           
if ($current_screen && ($current_screen->base == "edit" || $current_screen->base == "post")) {
    // Add-or-edit-page/post-only code       
 }