Microsoft Online Services team published a white paper about Public Folder migration to BPOS. By the way, Public Folders are not supported which means that this documentation will explain how to work around and use Sharepoint Online Services.
Read more at source: http://blogs.technet.com/msonline/archive/2009/07/16/microsoft-online-services-publishes-public-folder-data-migration-white-paper.aspx
Download it from here: http://www.microsoft.com/downloads/details.aspx?FamilyID=07bed889-7ee1-42fa-96b1-e03deef18ce5&displaylang=en