convert OU to domain

We sometimes have instances where sub-companies (OUs in the present AD structure) in our domain might need to leave the company umbrella.  Setting aside the question of whether they should have been setup as child domains in the first place, is there a way to convert these OUs to child domains?  Each site has its own set of DCs and Exchange server and its own user, groups, OU, etc. substructures that we would like to retain.  We would also need to setup two-way transitive trusts between the DCs, of course.  As far as Exchange goes, the mailboxes for the site/OU members are mostly on the site Exchange server or can be moved easily enough.  Still, are there any other Exchange considerations that we need to think of?  Has anyone done something like this before?  Any checklists, how-tos, recommendations, other resources out there?