I was looking for references on how to migrate mailboxes CrossOrg in Exchange 2007 and found these interesting articles (All from the primary source of Exchange info You Had me At EHLO)
Exchange 2007 migration overview
The process of migrating mailbox content between Exchange servers has been greatly improved for Exchange 2007. Differently than previous versions, our tools are now part of the server code and are therefore more integrated and functional (Covers Intra Org moves).
Exchange Server 2007 Export Mailbox task
Export-Mailbox is a task developed by the migration team to allow Administrators to export content from active mailboxes to a folder inside other active mailboxes. The initial idea for this task was to be a complete replacement for ExMerge. The implementation of some of this functionality was problematic and it required more time than initially planned. In this post I will describe what we are missing, which workarounds are available, and some of our future plans. (Contains all the excuses as to why PST export/import isn't supported until SP1).
Exchange 5.5 to Exchange 2007 mailbox migration
Migrating mailboxes from Exchange 5.5 to Exchange Server 2007 directly is not supported. Using native tools, administrators will have to perform this kind of migration as a two-step process which includes:
1. Migrating from Exchange 5.5 to Exchange 2003
2. Migrating from Exchange 2003 to Exchange 2007
Exchange 2007 Cross Org Mailbox Migration
Cross Org migrations are the ones where a mailbox needs to be moved from one Exchange Organization to another. Since there can only be one Exchange Organization per Forest, that means moving the mailbox between two different Forests.
Exchange Migration Wizard was used to perform this task in Exchange 2003. Exchange 2007 has incorporated Cross Org migrations into the server code base, so now Administrators can perform these moves by using the same task used for Intra Org migrations: Move-mailbox.
Happy reading ;-)