Two new technical articles just posted on MSDN:
Title: Upgrading SharePoint Portal Server 2003 Customizations to SharePoint Server 2007 (Part 1 of 2)
URL: http://msdn2.microsoft.com/en-us/library/bb954662.aspx
Title: Upgrading SharePoint Portal Server 2003 Customizations to SharePoint Server 2007 (Part 2 of 2)
URL: http://msdn2.microsoft.com/en-us/library/bb954661.aspx