We're getting ready to migrate our public folders from Exchange Server 5.5 to Exchange Server 2003. Can we prevent users from posting content to folders during the migration?

Yes. Add replicas of your folders to the target server. The built-in public store­replication mechanism will immediately start copying data to the new replicas. Replication will continue when you remove the folder replicas on the source server—no data will be removed from the source server because replication will still be in progress. However, users won't be able to post to the existing replica (because it's been removed) or to the new replicas (because they're still being replicated).