From the SSIS development team, Matt Masson has posted a few blog posts on how to use the SSIS API. The posts use references to the 2008 version of SSIS, but to modify them for 2005 requires a simple change in most cases – Upgrading custom SSIS 2005 components to 2008.
You can find the blog posts here: http://blogs.msdn.com/mattm/archive/2008/12/30/samples-for-creating-ssis-packages-programmatically.aspx
Also, there is a post on using a new API framework for SSIS 2008, titled EzAPI: http://blogs.msdn.com/mattm/archive/2008/12/30/ezapi-alternative-package-creation-api.aspx
I haven’t looked into the EzAPI yet, but it certainly sounds interesting.
Let me know what you think about the new posts and if you’d like to see any thing else from the dev team and I’ll pass it along.