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Hi there

There is no such thing as a simple FE/BE setup anymore im afraid , although in a way you could say that a Hub with Mailbox backend / CAS is this.

Exchange 2007 is split up into roles

- Mailbox (MB): The Mailbox server role is responsible for hosting mailbox and public folder data. This role also provides MAPI access for Outlook clients. Note that there is also a variation of this role called Clustered Mailbox role, for use with high-availability MSCS clustering of mailbox data. When Clustered Mailbox role is selected, other server roles cannot be combined on the same physical server.

- Client Access (CA): The Client Access server role provides the other mailbox server protocol access apart from MAPI. Similar to Exchange 2003 FrontEnd server, it enables user to use an Internet browser (OWA), 3^rd party mail client (POP3/IMAP4) and mobile device (ActiveSync) to access their mailbox.

- Unified Message (UM): This role enables end users to access their mailbox, address book, and calendar using telephone and voice. IP-PBX or VoIP gateway needs to be installed and configured to facilitate much of the functionality of this server role.

- Hub Transport (HT): The Hub Transport role handles mails by routing them to next hop: another Hub Transport server, Edge server or mailbox server. Unlike Exchange 2003 Bridgehead that needs Exchange admin defined routing groups, Exchange 2007 Hub Transport role uses AD site info to determine the mail flow.

- Edge Transport (ET): The last hop of outgoing mail and first hop of incoming mail, acting as a "smart host" and usually deployed in a perimeter network, Edge Transport provides mail quarantine and SMTP service to enhance security. One advantage of this role is that is does not require Active Directory access, so it can function with limited access to the corporate network for increased security.

More in-depth information about the roles can be found

Getting Started with Exchange Server 2007: Server Roles

http://technet.microsoft.com/en-us/library/aa996031(EXCHG.65).aspx

You will need to treat it as two phases:

1.       Coexistence

2.       Upgrade.

During phase 1, you will need to setup a new forest and add Exchange 2007 to it. At this point you have a separate instance, working completely independently.

During phase 2, you will need to connect the two forests and migrate the users over from the old domain.

The problem is that the old domain is Exchange 2003 and you are moving to a dual forest. This is covered here: http://technet.microsoft.com/en-us/library/aa996926(EXCHG.80).aspx

You will need to make sure you plan and test the migration before attempting it

Upgrading to Exchange 2007

http://technet.microsoft.com/en-us/library/bb124008(EXCHG.80).aspx

It all comes down to planning planning and more planning and then its then  into testing test and more testing before touching your live environment

If you are looking at a cross domain migration, then they need to consider very carefully where you want to end up and how long it may take to get there. I would highly recommend Exchange 2007 training, before attempting any migration to Exchange 2007.

A good source of information is the Exchange team blog

Microsoft Exchange Server 2007 and Windows Server 2008

http://msexchangeteam.com/archive/2007/08/16/446709.aspx

http://msexchangeteam.com/archive/2006/11/02/430289.aspx