I was thinking of all the private information that I never have time to secure delete from my trash because I'm too impatient, and I thought it would be a good idea just to run Disk Utility's Erase Free Space (35-Pass) feature overnight just for good measure. Unfortunately, it was not done in the morning, so I hit Skip, and I was in a rush, so against better judgement, I then used Force Quit. However, Disk Utility erases free disk space by simply making a giant file that is nothing but zeros or random data. It left that huge file, leaving me with 268Kb of free space, instead of 70Gb. What's worse is that I had no idea where it left that file! A reboot didn't clear it out, and I couldn't find any help on the usual Mac sites ... so I started messing around.
It turns out that the fix is simple: just throw some file away and empty the trash, and you'll get the space back. If for some reason you can't find a file to throw away, open Terminal and type touch ~/Desktop/throw_me_away. This will create an empty file on your desktop you can then delete.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070327052857103