If you have enabled Time Machine, the disk space may be taken up by local snapshots. Local snapshots are enabled by default for laptops and disabled for desktop Macs. When they are enabled, Time Machine saves hourly snapshots to
/Volumes/MobileBackups/ when a backup disk is not available and there is more than 10% free disk space. The local snapshots are not counted as used disk space in Finder, but they are classified as backups in the About This Mac window. See http://support.apple.com/kb/ht4878 for more information.
If it's not that, you might have the same issue I had with my MacBook Air. It was missing about 20 GB of disk space that was classified as other in the About This Mac window and shown as hidden space by Daisy Disk. When I tried verifying the Macintosh HD volume in Disk Utility, there was an error about an invalid free block count. I was then able to free up the disk space by repairing the disk from the recovery partition.
To see if the space is actually taken up by some files, you can use
$ sudo du -h -d1 /
-d1 sets the depth to 1 and
-h uses human-readable file sizes.