A couple of things I found by doing some searching:
MBS FileMaker Plugin
ODBC Driver for LDAP
Article related to using PHP to access LDAP
SmartPill FileMaker Plugin
Hope this might be of some help.
I'll second BowdenData's recommendations.
I've used SmartPill to query LDAP during a startup script to mimic LDAP user properties (department, group membership, etc.) in a FileMaker user table; allowing enterprise security management to dictate user properties in FileMaker.
You can get there from here!