Here's our current environment:

Forest 1:

Running Exchange 2003 in the parent domain with 6500 mailboxes.  Active
Directory is Windows 2000.

Running Lotus Notes in the child domain with about 1000 users.

Forest 2:

Running Exchange 5.5 in a Windows 2000 Active Directory domain with
about 500 users.

What we want accomplished is to consolidate all of our messaging
systems to one system using Exchange 2003 in a new Forest.  We are also
planning on consolidating all of these domains to this domain next
year.

So what we want now is to have one messaging system in a new domain but
the users would still logon to their current domain for file shares /
printers.  These same users will be migrated to the new domain that
Exchange would be placed in a year from now.  Is this possible and if
so how is the best way to approach this?  Would there be any 3rd party
tools that we would need since we have Exchange 5.5 and Lotus Notes?