I got this error the other day on my LDAP server.

warning: cannot open /etc/hosts.allow: Too many open files
warning: cannot open /etc/hosts.deny: Too many open files

I found a suggestion here: http://lists.centos.org/pipermail/centos/2009-October/084285.html

 Simple fix is to add this: 

*               -       nofile          10000
to your /etc/security/limits.conf file

Then restart your LDAP daemon via the init script.  (You can change the value 10000 to whatever you wish.)

 This seems to be holding up. Just thought I would repost here to make it easier to find ;)

-E