If you have already planned a complex Exchange Server 2010 transition between forest, multiple domains and that fun stuff probably you've found in your initial research the words MIIS and FIM which is the new name of the product and it has been incorporated in the Forefront Family.
So this video will explain the concept of inbound and outbound synchronization.
The ability to manage distributed identity information from a central point is key component of the Microsoft Forefront Identity Manager (FIM) 2010 architecture. This process is governed by a well-defined and customizable set of synchronization rules. In this video, I'll introduce you to the central concepts of inbound and outbound synchronization in FIM.
Check this video out at http://technet.microsoft.com/en-US/edge/Hh289368(MSDN.10)?query=1