In SharePoint 2010 there is this great new functionality to define a Site Collection as a Content Type Hub. Once you do this, SharePoint will copy any Content Types you declare in this Site Collection out to any other Site Collections that subscribe to the same Managed Metadata Service as the Content Type Hub.
Today a client asked if they can define Content Types at sub-site levels in the Content Type Hub site collection and have them published out. Interesting idea, and there was nothing that confirmed or denied it in the TechNet documentation. So after some testing I can now confirm…that you can’t. Only Content Types defined at the root of the Content Type Hub Site Collection are published out.