Jingmei Li has put together a great post detailing partner solutions supporting SharePoint Portal Server 2003 to Microsoft Office SharePoint Server 2007 migrations; these solutions are useful in situations where any of the three supported approaches do not fit within the scope of a business' plans for migration and upgrade.  Read the entire post here http://blogs.msdn.com/jingmeili/archive/2007/03/13/evaluate-partner-solutions-that-support-migration-from-sps-2003-to-moss-2007.aspx.