Lately i've run into a lot of people with very little available disk space, due to the fact that they have a significant number of AIFFs in their iTunes Library. I usually tell them that they can create a Smart Playlist that finds all of their AIFFs and then select all and convert them all. Afterwards, they can select all of the songs in that playlist and Option-Delete them to remove them from the Library with the option to delete them from disk. However, this isn't always a viable solution because sometimes they will be lacking the disk space to do the conversion in the first place.
So I wrote this AppleScript that singles out all of the files in your Library and converts them to a format of your choice, but it deletes each old song (from the disk and from the Library) as soon as that song is done encoding. Thanks to Doug Adams, as I looked at one of his scripts to figure out the 'choose from list' thing.
[robg adds: I haven't tested this one ... but as a general comment, if you're so hard-pressed for disk space that you can't convert then de-duplicate, you're going to have other system-related issues as well (assuming your music is on your boot disk/partition). Keeping at least 10% of your drive space free is advice that I've followed for quite a few years; I don't know if that figure is too high or too low, but I've never run into any drive-space-related issues using it as a benchmark.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050714154930216