A variety of third-party utilities can display details about the track that's currently playing in iTunes. But Hints reader Keir-thomas discovered a secret preference setting built into OS X Lion that allows song notifications to pop up over the iTunes Dock icon. Here’s how to enable them:
First, quit iTunes if it’s already running. Fire up Terminal (from /Applications/Utilities/) and paste in the following command, then hit Return:
defaults write com.apple.dock itunes-notifications -bool TRUE;killall Dock
The first part of that command turns on the Dock’s ability to display iTunes notifications; the second part restarts your Dock. (That restart may take a couple of seconds.)
Now, relaunch iTunes, select a track, and press play. Down in the Dock, you’ll see a brief notification with the song’s name and artist. The notification will appear whenever the track changes, regardless of the app you’re currently using; it will then fade away on its own.
If you decide you don't like the notifications, you can get rid of them by issuing the same command, but replacing TRUE with FALSE.