I don't know any boards that support ldap, but I know that vBulletin 3.5 has a very extensive API, maybe you can hack something away from that.
webg
2:25 pm on Oct 27, 2005 (gmt 0)
Pretty much any of them could be finagled to use LDAP. There is supposed to already be a hack for Web Wiz Forums (ASP-based), according to their support forums.