B2B Managed Services and SaaS?

One of the key value propositions for B2B Outsourcing is the ROI gained fromcontracting for B2B expertise and infastructure at a lower TCO than keeping all the people and infastructure in-house.   I'm not going to re-hash ROI model here as the growth of Managed Services sales (and those selling these solutions) is increasing significantly.   Instead I want to think about what it would take to deliver an outsourced Managed Service using the SaaS model.