We came across this little nugget on MSDN that might be useful to some…
To add a content type to a provisioned site through the Windows SharePoint Services object model:
1. Use the AvailableContentTypes method to access the content types that are available for the site. This method returns an SPContentTypeCollection object.
2. Create a SPContentType object using the SPContentType method.
3. Use the Add method to add the SPContentType object to the site content type collection.
