At OpenWorld, I presented in a session titled "Role of SOA in Enterprise Architecture". Out attempt was to understand whether SOA = Enterprise Architecture? Is it possible to do EA without SOA?
To begin with, there are lot of similarities between the two.
a) They both are out to achieve similar goals. Enterprise Architecture is about aligning business objectives with IT strategy. SOA is built on similar premises.
b) Enterprise Architecture initiatives are predominantly driven in top-down fashion. Company executives, LOB managers, IT executives collectively define enterprise operating model and how enterprise architecture supports it. SOA too has higher chances of success with top-down approach vs bottom-up approach. How many projects have we seen where IT ends up building bunch of web-services with no serious uptake in the organization? SOA is called SOA, not SOT, for a reason
Enterprise Architecture has 4 major sub-domains: Business Architecture, Application Architecture, Information Architecture & Technology Architecture. I would say SOA can play a role in each of these domains. See picture below
However, SOA only focus in Enterprise Architecture could be detrimental. Not every problem in an organization could be solved with SOA. If SOA is the solution to every problem in your organization, I would be inclined to say that you are victim from SOA Golden Hammer. EA is much more than SOA. From IT perspective, it encompasses Business Intelligence, Security, Content Management, Collaboration, Virtualization, Cloud Computing etc.
SOA is one of the best tools for an EA project to succeed.