Esetleg: http://forums.macrumors.com/showthread.php?t=1310197
So I booted into the Recovery HD, and did a "Repair Disk" from there. I forget the exact verbiage, but it was something to the effect of "incorrect free volume blocks reported," or something like that. So after doing that and a permissions repair for good measure, rebooted into OS X and now Disk Utility, Finder, and all the other 3rd party programs are reporting the actual amount of free space. Not sure what would have caused 80+ GB (at its biggest size) to show up as "Hidden space."