You might want to take a look at AddressBook2LDAP and AddressBook4LDAP available on j2anywhere.com If you have problems with the applications there is also online support via iChat / email.
It might be a little tricky to set up but it does work.
What should I allow, web browsing and IM should be enabled for the big boss to be happy. In wich interface should I block?
Thanks and excuse my newbyness