This site has a great article on how to enable LDAP auth (AD) w/ subversion + apache running on a linux machine… http://www.jejik.com/articles/2007/06/apache_and_subversion_authentication_with_microsoft_active_directory/
Instructions here If system is on a domain, be sure to add the additional reg key to enable this properly. The instructions for this are listed in the comments section at the location above.
when you log into cygwin you will be prompted to generate an /etc/passwd and /etc/group file… however in large domain enivornments with tons of users and groups, this can take a long time or even hang.. here is a procedure to get this to work: http://www.cygwin.com/ml/cygwin/2005-06/msg00384.html