Traditionally, the Business Units are responsible for the Lines of Business (products & services) used to deliver value to consumers (customers who purchase / consume their products & services). The Business Functions exist to support and/or serve the needs of the Business Units (directly and/or indirectly) with variations of granularity (based on where they are situated within an organization where the visibility is seen thru functional breakdown). Due to the highly distributed nature of organizations and expanded scope of services that are now being delivered to customers, the lines are no longer crisply drawn between BUs and BFs. That's a good thing since businesses are discovering new ways of extending value of their internal services (especially from a B2B model). That may suggest that we need to consider shifting away from the way we perceive (and subsequently define) the business organization (BUs and BFs) and embrace businesses as a series of one or more supply chains designed to provide services within and without a given organization.
It's a more dynamic and organic business model that allows architects to define things within a "flow-based" service model (which is where most of our systems of systems thinking is moving) and away from the "discrete" functional models, which was derived from our industrial manufacturing mindset. This new approach also creates a more fluid perspective around planning and may create more innovation opportunities for the way organizations are conceptualized, managed, and operated.
I can just imagine how this might shift the discussion around the definition of architecture and that of the enterprise in future discussions.