If I install SQL 2008 on a W2K8 or W2K3 R2 cluster, can the individual
(non-primary) instances run as users in different child domains of the parent
domain?

What if the cluster is in a trusted forest? Can the instances run as users
in different trusted domains in different trusted forests?

Example:
Management domain 'owns' the cluster and the first instance of SQL (for
management purposes).
Domain A with application needs to be SA or equivalent domain user for a SQL
instance to run its application data.
Domain B with application needs to be SA or equivalent domain user for a SQL
instance to run its application data.

Do Domain A and Domain B need to be child domains of Management?
Can they be in different forests with appropriate forest to forest
transitive trusts?

Thank you for any insight you have.