This is interesting one…
There’s a hidden setting for the Dock that will show pop-up notifications of which iTunes track is playing, a little like Growl.
First, quit iTunes if it’s open, then open a Terminal window and type the following:
defaults write com.apple.dock itunes-notifications -bool TRUE;killall Dock
Then start iTunes and try playing a track. Neat, eh? The pop-up fades away after a few seconds. To add the iTunes icon to the pop-up window, type the following into a Terminal window:
defaults write com.apple.dock notification-always-show-image -bool TRUE;killall Dock
To deactivate the pop-up at a later date, quit iTunes again, then open a Terminal window and type the following two lines:
defaults delete com.apple.dock itunes-notifications
defaults delete com.apple.dock notification-always-show-image;killall Dock
My theory is that these pop-ups hint at either a forthcoming notification system (maybe in OS X 10.8), or it’s a legacy of a notification system that Apple decided to abandon. But it’s easily to imagine a similar system working with Mail, showing notifications of incoming mail. In addition to the preference keys mentioned above, I found various other keys relating to the height and length of the pop-up bubble. Whoever designed this clearly intended it to be tweakable.
From Thomas Keir, author of book Mac Kung Fu