I have a minimal installation of CentOS 5.7 32-bit on a VM.
The apropos command always returns "nothing appropriate"
[root@localhost ~]# apropos man man: nothing appropriate
Even after I run updatedb. It works on another CentOS server I have.
Try executing 'makewhatis' as root. That updates the man page database, and creates it if it doesn't already exist, which it may not on a fresh installation.
'updatedb' updates the database for 'locate', and probably isn't related to this issue.