iTunes track notifications for Mountain Lion
As best as I've been able to tell, this feature has been removed.

There are two alternatives to the Lion-provided notification mechanism that provide a few more features and customizability.

If you have Growl installed, you can use GrowlTunes to provide Growl notifications on track and playback changes.

The official Last.fm Scrobbler app provides notifications via Growl as well, the bonus being that it's worth using if you also scrobble your playback history to Last.fm. If you don't you probably want to stick with GrowlTunes.

Growl is a generic notification system with a large amount of customizability, like the ability to customize the alert style. Growl has existed for OS X for as long as I can remember, years and years pre-dating even Tiger, iOS, and the Notification Center as well.