Joe McKendrick writes an interesting post about if SOA is simply Software as a Service (SAAS) delivered internally. He makes some interesting points and while there are certainly parallels I think that ultimately they are different. Consider the following:

SOA - Two of the key benefits advertised by Service-Oriented Architecture are agility and reusability. In the past I have written that I don't buy the reuse argument at all, and think that mainly reusability benefits exist mostly in the infrastructure capabilities (monitoring, deployment, etc.) So if you discount the reuse argument then the key benefit of SOA becomes agility. The agility of services seems to actually be valid for two reasons:

1. SOA has caused the adoption of several standards for interfaces. Since many applications now support XML based messages for example integration becomes a much easier task.
2. SOA also focuses on business processes. This encourages businesses to break off individual capabilities that need to evolve rapidly to support changing business drivers. This is where SAAS comes in.

SAAS - The benefit of SAAS is that businesses can can use services developed and maintained by an outside vendor. Reusability actually works here because the provider of the service can solicit for customers much like any software vendor does. It seems to me this helps reduce costs for the customer since they don't have to bear the development cost themselves, but at the cost of agility. Since initially a SAAS vendor will have fewer customers they should be more responsive to change requests. Like all vendors though as the number of users grow their ability to react is slower. It is true of course that they can and must support multiple versions of their service, but this also increases the cost of providing the service lowering the value proposition for the customers.

So if SOA isn't highly reusable, which is the key benefit of SAAS, than SOA can't possibly be SAAS delivered internally.