This document is the list of numbers that have currently been assigned by the IANA. Sine Nomine Associates has been assigned PEN 17297.
To request a Private Enterprise Number for your organization, visit the IANA online application form at http://www.iana.org/cgi-bin/enterprise.pl. To see the list of already-assigned numbers, click the "visit..." link from this page (see below). You should always check to see if your organization already has a PEN before requesting a new one.
For information about PENs and LDAP, see http://sinenomine.net/faq/techreference/ldap.