Last night I started to delete 100GB worth of VirtualBox disk images on a MBP running Lion. A dialog box opened saying that the trash was preparing to empty, but nothing happened for a good 20 minutes. I had to shut the computer down, so I cancelled the process and waited another 10 minutes or so for the window to disappear. When it did, the trash appeared to be emptied, but I never got back that 100GB of space.
I've tried some of the suggestions from similar questions on this site, but so far nothing has worked. There's nothing in the Trash, in ~/.Trash, or in /user/username/.Trash.