iTunes sends out a notification when something changes so just register for it in your init method of AppDelegate. Here's an example...
[[NSDistributedNotificationCenter defaultCenter] addObserver:self selector:@selector(receivediTunesNotification:) name:@"com.apple.iTunes.playerInfo" object:nil];
The actual notifcation object in your method "receivediTunesNotification:" will contain information about the changes.