[4:53am, Shanghai - jet lag can be nasty sometimes]
I discovered an issue with iTunes a little while ago while performing a routine clean-up of my hard disks. Essentially, when iTunes tries to save your "iTunes Music Library.xml" file, it appears to create a temporary file first that it renames once it is successfully saved.
The issue is that iTunes doesn't clean up these temporary files, for some reason. When I didn't have many songs in iTunes, that didn't seem to matter so much, but now I have 65 Gigs of music (approximately 12,000 songs and counting) my Library XML file is 20Mb in size. Every temporary file it creates is the same size - 20Mb - and that soon adds up. iTunes is supposed to delete these files when you exit the software, but (a) it doesn't and (b) I never quit iTunes anyway as I leave it running to download PodCasts.
What is worse, is that it seems to be adding between 5 and 8 copies of this file every day. Here is a screen shot of my current iTunes folder. I cleaned it up 5 or 6 days ago. I am already back up to 1 Gig of used space in "Temp File X" files already.
I have had to set up a nightly job that emails me the size of this folder so that I know when to go and purge it. Ridiculous.
There is an Apple help file on this that you can find here: "iTunes: Multiple Temp Files in iTunes Folder". Incredibly, this has been happening since version 3 of iTunes. *Surely* iTunes knows when it isn't using a particular "Temp File X" and can just go and delete it for me? Oh well.
So, go and check your "My Music\iTunes" folder to see if you can reclaim any of your disk space!