More than likely taken up by local snapshots. Details in http://support.apple.com/kb/HT4878
It can't be due to local snapshots. Lion's Time Machine will delete local snapshots from an hours worth, to the whole thing, if disk space is needed by the OS or any other application other than Time Machine.
You need to figure out what that "other" stuff is. What I'd do is either run an exhaustive du on the system to find large files (such as sudo du -k / | sort -rn | head -10), which could take awhile, or download this software called "Daisy Disk 2" - http://www.daisydiskapp.com/
Daisy Disk 2 is annoyware (meaning that you have to wait about 30 seconds on startup before you can click on the "test out" button, unless of course, you purchase the software) but it's free from there. It's probably faster than using the command from above and it will basically give you a drill down from your largest files/folders on down so that you can see what is actually taking up space on your drive. I've used it to clean up alot of my system, from old music folders, to dmg images that I've created, all the way to downloaded dmg's that I've failed to cleanup (I really need to configure my Hazel installation). By the way, when you run Daisy Disk, I just uncompress the dmg image and run it from my downloads folder and I DO NOT move it into my applications folder. I'd run it as a utility every now and again, and for me, there is no reason to fully install it.
I'd give that a try and report your findings.
Again, it shouldn't be your local snapshots no matter what, even if they were enabled.