The legacyExchangeDN value is not determined by GALSYNC...........Exchange actually supplies this value. If using Exchange 2003, then the RUS (recipient update service) runs all the time and is responsible for that. If using 2007/2010/2013, then this needs to be invoked when making objects programmatically, the way MIIS/ILM/FIM does. ILM FP1/FIM use a powershell cmdlet (which actually is part of Exchange) called 'update-recipient' that performs this functionality. If you are seeing contacts provisioned but they aren't in the GAL and lack this attribute, then you either are using version of sync engine before ILM 2007 FP1 or you are not using 'Exchange provisioning' (configured on the 'configure extensions' property page of the GALSYNC MA) checkbox.
GALSync, in MIIS/ILM/FIM does NOT do ANYTHING with legacyExchangeDN attribute value, except for reading it from objects and synching it to the proxyAddresses attribute of connected objects.
It is Exchange that determines the value of the legacyExchangeDN attribute, not Galsync
Jorge de Almeida Pinto [MVP-DS] | Principal Consultant | BLOG: http://jorgequestforknowledge.wordpress.com/