Gartner recently published an article on migrating AD. They cover the complete process to migrate or restructure Active Directory for mergers, acquisitions, and divestitures. They also mention that Intra-forest migrations are typically destructive and inter-forest migrations non-destructive. The reason for this is that with intra-forest the objects are moved and with inter-forest the objects are copied enabling easier rollback. They also mention that vendors can make this process more seamless for end users and easier to perform.
Included inside the article is the mention of Quest Software and Migration Manager for Active Directory. Migration Manager can perform an inter-forest migration that is seamless for the end user, provides full object migration and resource processing, and provides complete rollback if needed making the whole process of migration easier.
The article is good and is a must read for those considering AD migration in the near future.