Together with the beta 2 of MOSS 2007 and WSS 3.0 Microsoft released a couple of nice documents and tools which should help MCMS customers to plan and prepare the migration to the new version on the SharePoint Server Migration Center
Articles and Tools
Learn to use the CMS Assessment Tool to determine the level of work required to migrate your MCMS 2002 site.
Download this tool to assess and analyze the work required to migrate your MCMS 2002 site to SharePoint Server 2007.
Understand the changes, differences, and benefits of the MCMS APIs as implemented in SharePoint Server.
Use this guide in planning your migration to SharePoint 2007.
See how SharePoint Server 2007 merges the functionality of MCMS 2002 and SharePoint Portal Server 2003 with new functionality to create one integrated set of technologies built on Windows SharePoint Services.
Learn about new features, tools, and resources that are available to help customers migrate their sites.
Look at the major improvements WCM offers over MCMS.
Read more on the SharePoint Server Migration Center