Via Joe McKendrick, I see that a Motley Fool writer has described SOA as "an extension of the software as a service concept to the internal IT departments of companies." Which of course it is. But you don't often see it described that way. People have got SOA pigeonholed as something you do as a grand scheme that operates at the heart of enterprise IT, deep down in the component layer, whereas SaaS is seen as something that happens at the periphery, at the business application layer.
Whereas the more complex truth is that both of these apparently separate movements are really concerned with a services-based approach to IT.Old application categories will fall apart and new ones will form Some of the services are like components. Others are like applications. A further bunch are like Web 2.0 information feeds and mashups. Another set are more like semi-automated business services that deliver real-world business results, such as shipping an order or transacting a payment.
The common unifying factor here is a services idiom, in which providers autonomously deliver results according to contracts. That describes SOA and SaaS and Web 2.0. SaaS denotes a service delivered by a third party, although since historically services have tended to be bundled up into applications, it's usually understood as meaning a software application delivered as a service.
The problem with that historical perspective is that it doesn't allow for the impact that services architectures will have on conventional application categories. Those categories arose in response to the constraints on IT at the time. Services architectures are now setting aside those constraints and therefore the old application categories will fall apart and new ones will ultimately be formed by combining services in new ways.
Or to put it another way, SOA and Web 2.0 will change the configuration of SaaS as radically as introducing SaaS is going to change the configuration of enterprise IT.