Migrating users from Exchange 2007 (on 2003 Domain) to new 2008 Forest and Exch 2010 Server

My company is in the process of planning an Active Directory redesign and migration from a native 2003 forest to a 2008 forest.  Currently we have two forests, one for our HQ office and one for our collocated datacenter (don't ask me why, it was here when I signed on).  What they want to do is put our HQ and Collocation AD under a single 2008 forest with the two sites being child domains of the root.  The planned design would look like this:

                          MyComp.com (Root)
                                      /  \
Persistent VPN -->   /      \
                                   /          \
       HQ.MyComp.com        Collo.MyComp.com
         (Child Domain)               (Child Domain)

Our current AD Looks like this (neither forests have site trusts):

HQ.olddomain.com  -------  Collo.olddomain.com
  (Separate Forest)      ^       (Separate Forest)    
                      (Connected via VPN )

My main task is to migrate the user mailboxes to Exchange 2010 that could either reside in the HQ.MyComp.com child domain or sit in the Root (MyComp.com).  I would like to know the pro's and cons of having Exchange in the Root domain and linking them to accounts in the trusted HQ.MyComp.com child domain.  Our Root and Collo.MyComp.com domain DCs and infrastructure are hosted in a much more robust location with replicating SANs and redundant network infrastructure, so that is why I would like to put the vital parts of our Exchange 2010 (Mailbox, CAS, Edge Transport) there rather than at our HQ location.  Disaster recovery would be much easier if the critical infrastructure was placed there.

The proposed plan entails migrating all HQ users from the old domain to the new HQ.MyComp.com child domain. I've been trying to find a best practices guide to this specific example but am having problems finding one that fits our end-scenario.  Any advice from the guru's of AD and Exchange would be of great help.
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its supported to have exchange in a separate domain - if you look up exchange 2010 resource forest

THe theory is the same but should be less effort

You should be able to share the same name space by making the domain non-authoritative on both servers and pointed them at each other so that you can bounce the mail between them
can you please state your questions clearly as from what i've understand, you need a full design for your new infrastructure plus the pros and cons of having exchange in the root domain.
with exchange 2007 and 2010 you can do cross forest migrations

I wouldn't put the exchange org in the root domain - try and keep that as clear as possible
Are you keeping the same email addresses?
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xtheory79Author Commented:
Revised question:  What is the best way of migrating from Exchange 2007 to Exchange 2010 in a new forest?  The forest itself will also be migrated from an old 2003 AD to a new 2008 AD domain.  The name of the new forest will be different from any of the ones.  Email addresses will remain the same.  I'd like to place Exchange in the root domain for redundancy and disaster recovery reasons, but would like to know what the pro's and cons are of having it run on the root.  

One caveat I have read was to not run the ADMT tool to migrate users before making the mailbox Move-Request to migrate the mailboxes to the new domain.  
xtheory79Author Commented:

Our plan is to migrate all users and Exchange mailboxes to a new forest.  Although I know it's possible to make one of the child domains a Resource Forest, I would like one Exchange server and all of it's server roles located in a single domain for DR and management reasons.  It's not necessary in my company to isolate mailbox stores or bounce mail between the child domains.

You mentioned that you would want to keep your root domain clear.  Although Exchange would be the only other service running within that domain, is there any substantial reasons as to why you would not want to run Exchange in the root, such as for security, replication issues, etc?
just following guide lines - security mainly, delegation of control to limit Admin access to change things but then my work is security conscious MS recommendations. That way if you do things that alter a domain you will still have the route domain clean.

the bouncing mail was for the migration, if you migrate over several days/weeks then you will need that to ensure you mail still flows.

One further thought why bother doing any proper migration export and import the mailboxes via PST's or something that way you can save taking any crap over.

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