There appears to be an issue with .Mac synching in Leopard which occasionally means that the disk image it syncs your iDisk to gets bigger than 1Gb… a lot bigger, in fact. In my my case, it suddenly decided that it wanted to claim 45GB of my drive space.
If this happens to you, turn off iDisk syncing. This will move the disk image to your desktop, where you can happily delete it. Leopard keeps its sync disk images in a different place to Tiger – /Library/FileSync.
Incidentally, I found this out by poking around in a fantastic little utility called WhatSize, which I’d heartily recommend to all.