6. In what order should I deploy Exchange 2010 roles?
Installing a single Exchange 2010 server with all four major roles (the Client Access role, the Hub Transport role, the Unified Messaging Server role, and the Mailbox Server role) within the domain resolves the issue completely, no installation order necessary. However, some organizations will want to go with a more staggered approach to deployment. In that case, the order is Client Access first, then Hub Transport, then Unified Messaging Server (if applicable), and finally Mailbox Server.
There's a reason for this order: major changes in how each server role works compared to Exchange 2007. In Exchange 2010, the Client Access and Hub Transport roles are needed for the Mailbox Server role to even function, and the Exchange 2010 Mailbox Server role cannot use any other transport server besides an Exchange 2010 Hub Transport server. You will need to put at least those three roles in place, move the mailboxes over, and then (if desired) decommission those legacy roles in your organization. Thus, you will install these server roles in parallel with your existing servers.
A solid piece of advice from the Microsoft Exchange team: If you are dealing with a multiple-site transition, start with one ActiveDirectory site at a time, beginning with your Internet-facing sites. Additionally, you should switch your host names from Exchange 2003/2007 to your new Exchange 2010 servers. Those Exchange 2010 servers will know how to handle the requests for the older versions, but the older versions will not know how to work with Exchange 2010. You will also need to provide for a legacy host name so that Exchange 2010 knows how to work with the legacy systems.
Once you have the entire structure in place, you come to the most exciting process: moving mailboxes!
7. What is an online mailbox move?
In previous transition scenarios with Exchange, moving a mailbox would temporarily take the mailbox offline, rendering it inaccessible and potentially locking users out of e-mail for a long time. With Exchange 2010, new cmdlets (do not use the old Move-Mailbox cmdlet!) will move the mailbox without disconnecting the client, but note that users running Outlook 2003/2007 will have to restart Outlook once their mailboxes are moved.
Note: With Exchange 2007 we used the Move-Mailbox cmdlet but in 2010 we use the Get-MoveRequest cmdlet. In addition, it is good to note that moving mailboxes from 2003 to 2010 will not be online, those are still offline moves.