I have in the same Exchange Organisation Ex5.5 and Ex2003.
Our Windows 2003 is In 2003 Mode (native)
I can Move mailboxes from 5.5 to Ex2003 without any problem
When I replicate Public Folder from Ex5.5 to Ex2003 the securtiy in Ex2003 Public Folders is not working.
RQ: Security in 5.5 is by Distribution LIST.