A good reference for people who work on ISA/TMG

http://www.ietf.org/rfc/rfc2616.txt

This is my new post and it is on the ISA/TMG product team site

http://blogs.technet.com/b/isablog/archive/2010/08/06/unable-to-authenticate-using-fba-with-ldap-on-isa-server-2006.aspx

and it talks about authentication problems if LMCompatibityLevels are not as required. Please have a look.