Server based systems such as ERP, CRM, email, database and collaboration are well understood as single standalone applications today. They provide the highest levels of service, reliability, scalability, availability, security and integrity and are what enterprises run on. Enterprises will continue to require the highest levels of security and data integrity that only server based systems can provide and so there will be the need for server based systems for the foreseeable future.
It is clear however that not all users or services require the highest levels of security, availability and integrity; the CEO of an organization has different needs of an email system than a factory floor worker, the CIO requires different application access than a part time agent, a salesman in the field needs different levels of Sales Force Automation support than a person in technical support.  In many cases it is both beneficial and cost advantageous to outsource these users and applications to an externally hosted or provided solution such as a hosted or managed service or even a full Saas service.
We need to think about how this server / service SLA dichotomy will effect the design of both the SaaS and the server systems going forward. For example the present server systems will have to recognize and work seamlessly with not only the present client based systems but also the managed and SaaS based systems in a federated style which gives a single enterprise image. The CIO and the part time agent will want the same email domain even if their mailboxes are hosted in different systems in different organizations thousands of miles apart.
Server systems will have to support federation not only at the organizational level but also intra organization and even into the consumer base. They will have to do this at the security level, the directory level, the email level and even the application level. This is the real challenge of SaaS to the enterprise.