Under some unknown circumstances (I haven't been able to consistently duplicate the issue), it happens that the system's disk icon vanishes in Leopard. Toggling the Show Hard Disks setting in Finder's preferences has no effect, and the various repairing tricks do not solve the issue.
What happens is that the system disk (by default, named Macintosh HD) becomes marked as a hidden (invisible) file. Why? Who knows. But fortunately, it can be reverted to visibility with a simple Terminal command:
sudo chflags hidden "/Volumes/NameOfTheVolume"
To make this work, change NameOfTheVolume to the actual name of your system disk. Note that the chflags can be useful for hiding/unhiding regular files and folders, too:
$ chflags hidden "/path/to/file"
$ chflags nohidden "/path/to/file"
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081112133835450