A big part of development against Exchange 2007 will be understanding where to put your code or which Exchange roles affect your code. Exchange 2007 can be broken out to run these different roles on different machines or all roles (except the Edge server role on one server). The Exchange team blog has a new post providing information on each of these roles.
Here is an excerpt of article introducing each of the 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), 3rd 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."