http://au.autodesk.com/sessions/detail/651/
http://www.adskconsulting.com/adn/cs/api_course_sched.php you can download the ADN class webcast that covered the Civil 3D API which should have had a section on creating subassemblies (I haven't watched this one though).
Also I found if you add the custom subassembly to an existing tool catalog you don't need to go to the trouble of creating a registry file.
Christopher
http://blog.civil3dreminders.com/
Civil Reminders
http://blog.civil3dreminders.com/
http://www.CivilReminders.com/
Alumni