Coghead CTO Greg Olsen makes some interesting points in this post "How Not to End up as an Anachronism" on the move from on-premise to SaaS to SaaS-on-SaaS applications.
The move to SaaS applications built on SaaS is a much more profound shift than the move from on-premise applications to SaaS applications. The software industry is beginning to display characteristics that mimic the supply chains and service layering that are commonplace in other industries like transportation, financial services, insurance, food processing, etc. A simple set of categories like applications, middleware and infrastructure no longer represents the reality of software products or vendors. Instead of a small number of very large, vertically integrated vendors, we are seeing an explosion of smaller, more focused software services and vendors. The reasons for this transition are simple: It takes less capital and other resources to create, integrate, assemble and distribute useful software capabilities.While the likelihood of enterprises abandoning all internal infrastructure is still low (at the moment) it's becoming a lot less appealing to have to purchase and manage large infrastructures. That said, I don't see the Fortune 50 moving everything to the cloud anytime soon.