Desiging complex Exchange and Active Directory networks can be difficult. All sorts of different issues comes up that require a lot of planning and testing. This is especially true when migrating from an NT environment (are those still around? I guess so) where consolidation of multiple Exchange 5.5 sites, isolated Exchange 2000 organizations and NT domains might occur.
The following article discusses the pros and cons of implementing a single AD site for Exchange 2003 in a large environment. This can save you a lot of lab time and guide you in your planning.