SOA doesn’t replace Enterprise Architecture, it’s simply one view of the enterprise. Today, one could argue that most organizations view the IT landscape as a collection of applications. Efforts like application rationalization or application portfolio management reinforce this notion. So, you could also say that today we have application oriented architectures. The unit of composition is the application. This isn’t flexible enough, as it is too coarsely defined. If we break these applications into smaller units, we now get to service oriented architecture, which I feel is a better way of describing things. Is it the only way? Certainly not.
Also, here are some links on the background of Enterprise Architecture that I think could offer some insight:
http://msdn2.microsoft.com/en-us/library/bb466232.aspx
http://www.enterprise-architecture.info/Images/Documents/Comparative_Survey_of_EA_Frameworks.pps
http://www.bptrends.com/publicationfiles/12-03 NL Enterprise Architectures.pdf
http://www.e-cio.org/lea_presentations/11_models_framework_files/frame.htm
