From Boneware :
LDAPeople is a versatile LDAP addressbook client. You can use it to query LDAP based directory servers for addressbook records. LDAPeople displays a list of entries that match your query and provides a detailled view for each record found. In the detail view, you can tap on a persons phone number, e-mail address or url and your device will instantly call that person or open your e-mail application or webbrowser for the selected address. You can even copy entries from the LDAP directory to your local Contacts database.
LDAPeople supports both LDAPv2 and LDAPv3 using anonymous bind or simple authentication. Everything that depends on attributes specific to a certain LDAP directory server is configurable. You are free to configure a custom LDAP filter for queries and there's even a complete attribute mapper that maps attributes from your directory server to iPhone addressbook attributes according to your needs.