I'm planing a new Active Directory structure for a multi-national company that consists for six very different business units and need some help, advice, or pointers in making this implementation happen. Each business unit currently has its own Windows NT 4.0 domain and a full complement of IT staff (administrators and help desk). What type of structure can I choose for this organization and why? Also, what would be the possible implications of this design vs. the alternatives? Thanks in advance.
Pick up the following book from O'Reilly: Active Directory, Fourth Edition ISBN:978-0-596-52059-5
after you read that and if you still have questions, please start looking for a qualified consultant.
The book covers AD 2000 to 2008. You may also want to study the MS certification books on AD. If you don't know/remember details about NT4 domains, get a book or two on that as well. Lastly, make sure you know DNS inside and out. If you need to use LDAP or BIND, learn about them too before you start planning.
There is no one answer and always depends on the detailed specifics of your organization.