One thing we keep seeing is where customers are still using MIIS 2003, but are upgrading their Exchange environments to Exchange 2007 and/or Exchange 2010.  Here is some great information on the topic.

GALSYNC INFORMATION: LegacyExchangeDN is not populated: http://social.technet.microsoft.com/wiki/contents/articles/7773.galsync-information-legacyexchangedn-is-not-populated.aspx