Robert,
The difference between partnering with Exchange 2007 is that Unity doesn't have the ability to create the system mailboxes like it did with Exchange 2003. So, a script is automatically generated by the Message Store Configuration Wizard (MSCW) and placed on the desktop of the Unity server. This script needs to be copied over to the Exchange 2007 server and ran through the command shell which will create these mailboxes. Without the successful execution of this script, MSCW will not allow you to continue.
Hope this helps. If so, please rate.
Brad