Administration of Active Directory does not have to be hard. Too often what should be a simple task is made more difficult than it needs to be.The solution? Hyena from SystemTools Software. With ease-of-use as well as powerful importing and bulk updating capabilities.
Check this link
http://www.microsoft.com/technet/prodtechnol/exchange/2003/multiforest.mspx
( Read the topic Active Directory and Exchange Server 2003 ). I hope this would give you some assurance.
Raja