Not too much to add, but I'd like to be notified of when a content type has been added or removed from the content types associated with a list. Is this possible?
I believe that there are no straight forward methods to accomplish this, because there are no events triggered when a content type is added or removed. You can also only attach workflows to items for specific content types.
Event handlers and workflows can only be created or associated to items on a list. Using SPWorkflowAssociation method, you can associate workflows to specific content types. But even these workflows are triggered, when items are added or removed to the list.