When you play a song in iTunes, there's a progress bar on the top of the window indicating the time played and remaining of the song. The remaining time always displays in a format such as -2:45. To change it, click on that negative time, and you'll see the total time of the track.
[kirkmc adds: While this isn't new--at least I think it's been around for several versions of iTunes--I couldn't find it on the site, nor in iTunes' help.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061020091233102