The SQL Server 2008 Books Online page in which we describe the steps for upgrading and redistributing custom components is now live:
Upgrading Custom Objects for SQL Server 2008 Integration Services
This topic is for shops that wrote a custom component for SQL Server 2005 Integration Services, and who now want to provide a new version of the same component for SQL Server 2008 Integration Services.
If you choose to create a new version of your component for 2008, then in addition to recompiling against the version 10 assemblies, you need to redist your component along with an XML mapping file that tells the package upgrade engine how to map the 2005 version of your component, that it finds in the 2005 source packages, to the 2008 version of your component, to reference in the upgraded packages. The mapping file format is simple and straightforward. I have uploaded the XSD schema for the file here, but the BOL topic also contains a simple example.
Integration Services developer and blogger Matt Masson worked on this functionality, and has blogged about it more than once. The BOL topic does its best to summarize all the most up-to-date information, but you may want to re-read Matt's blog posts on the subject as well:
Please let me know if you find the BOL topic incomplete or unclear! You can always write to me here (dougbert-at-dougbert-.com) or use the Give Feedback link that appears in every single BOL topic. Thank you!