New in Exchange 2003 SP1 - The ability to consolidate sites even from 5.5 to Exchange 2003!
It is also recommended that all your Public Folders be centralized already. After consolidation there is a tool called exprofre.exe that will fix your Outlook profiles. Mailbox rules will continue to work after the move. DL membership will continue to work.
Cool, huh? Makes you want to know when SP1 will be released huh? Soon... Real soon...