To enable this, start by opening Terminal (located in Applications/Utilities), then paste in the following line and press Return.
defaults write com.apple.dock itunes-notifications -bool YES
For the changes to take effect, the Dock has to be restarted. The easiest way to do this is to type killall Dock into the Terminal and press Return. If you change your mind and want to disable the notifications, repeat the above commands, but replace YES with NO.
For those of you who have your Dock set to hide when not in use, you’ll notice that when the Dock is hidden the iTunes icon is added to the notification. If you want the icon to show even when the Dock is not hidden, just use the following command in the same way as above:
defaults write com.apple.dock notification-always-show-image -bool YES
As before, remember to restart the Dock with killall Dock for changes to take effect.
It’s not entirely clear why this setting isn’t present in the iTunes preferences. It might still be under development — there is still room for improvement. For example it would be better if it included the album artwork like Bowtie, Growl and DockArt already do. It could also be a sneak peak of future plans Apple has for a notifications framework for all apps in Mac OS X, much like Growl provides at the moment.