I couldn't locate any information on Apple or elsewhere on what the proper format for the LDIF file Address Book was expecting, so I thought I would post what worked for me:
dn: cn=David Martin,mail=davidfmartin@yahoo.com
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: inetOrgPerson
givenName: David
sn: Martin
cn: David Martin
mail: davidfmartin@yahoo.com
TelephoneNumber: 303-555-1212
FacsimileTelephoneNumber: 303-555-1313
Pager: 303-555-1414
Mobile: 303-555-1515
homePostalAddress: 17/275-09
ou: 3100
O: You company name here
description: Mailstop - 12275, any addl. comments.
Once I knew the format, it was then a simple task to create a template I could use in a Perl program that would convert a proprietary contact database to LDIF, and then import that into Address Book.