If your implementing Exchange 2007 in a pre-existing 2003 Domain it will work although it is not offically supported by RIM. You will require the MAPI & CDO component from Microsoft. Download details: ExchangeMapiCDO
Other then that the permissions for the BESAdmin account will work fine for Calendaring.
In regards to a NEW Exchange 2007 forest you can implement a reg hack to apply the permissions needed from a Domain level within Active Directory. As anyone that has been testing with 2007 all permissions have changed into Role Administration level, Microsoft left out a service account for RIM/Goodlink.
**Note: I can not comment on behalf of RIM as I am strictly an IT Administrator which likes to push customer products to their limits.