Migrating from Exchange 2003 to the Exchange 2007 platform is often under-estimated. In fact, the migration work required here can be more technically challenging than migrating off another email system (eg- Novell GroupWise).
The visibility to the end-user is minimal, as they continue to use Outlook with their existing MAPI Profile, so you will get no thanks for completing the project successfully. However, if you slip-up and make a mistake, then the user population will be very unhappy.
This project has the advantage though, of allowing for a longer co-existence period, as the user experience will be very similar, irrespective of which whether they are on Exchange 2003 or Exchange 2007. For that reason, you are able to plan the migration very carefully, and take your time performing all the required steps.
If possible, you can time this project to coincide with some other new technology that has high visibility to the user. This could be Outlook 2007, a refreshed workstation, or Office Communicator 2007. This way, you will get the users on your side for the project.
Top Tip: Take a CSVDE Export of your Active Directory before any mailboxes are migrated so you have a record of who was homed on which Exchange 2003 Mail Store. This is required if you are asked to do a restore of a mailbox from Exchange 2003 from after the migration is completed. The CSVDE export file will contain the Home Mail Store info you need. See the Blog section for information.