For a long time now, I've wanted to convert my "old" and "outdated" MP3 files into AAC format -- mostly just to free up some space. I have put this off because manually doing this for every song seemed like such a massive undertaking that it didn't even seem worth it. Automator makes this task pretty simple.
First off, I downloaded some additional iTunes Automator scripts from Doug's Scripts. The one we're specifically looking for is the "Convert Tracks" action, which allows you to convert tracks and delete the originals afterwards.
Although you can do this completely within Automater, I chose to use iTunes to create a smart playlist called "MP3s" that matches based on 'Kind is MPEG audio file,' which will automatically separate all of your MP3 files into one playlist. You could also use 'Kind is not AAC audio file' if you have file types other than MP3s in your collection that you want to convert. An interesting note is that doing this causes "Protected AAC audio file" types to show up as well (those purchased from the iTunes Music Store, for example), but these will not be converted because (obviously) they are technically already AAC files.
Once you've got your playlist set up, the Automator action is only two steps:
Mac OS X Hints
http://hints.macworld.com/article.php?story=2005061722325955