If you, like me, don't relish the task of re-ripping your entire CD collection after a hard disk failure, then you've probably enjoyed iTunes4's CD/DVD playlist backup facility. However, keeping track of what you've backed up can be a pain. Here's a simple solution.
After your first full backup, create a smart playlist with the following condition: Date Added - is after - "the date you last backed up". Over time, the smart playlist will fill up with songs you've added until it reaches a size suitable for burning to CD/DVD.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030515063633847