I think that Charles Zedlewski gets it exactly right in his post A new wrinkle on SaaS when he says this about "Salesforce to Salesforce":
This is something that would be difficult for on-premise vendors to replicate: an application network effect that's could be genuinely beneficial to the user.
This will be a powerful feature as companies with an indirect business model begin to leverage it with their partners.
Just think how hard it would be to implement this functionality with an on-premise solution - the logistics of coordinating IT organizations would be monumental.
