[Two excellent articles on the role of Web 2.0, SOA and AJAX - some excerpts -- BSA]
There is no Web 2.0 without SOA
For Web 2.0 to really deliver the hard work is getting the existing systems ready, not in the flashy GUI.
The point here is that not only do SOA and Web 2.0 work well together its actually really hard to see how you can have enterprise grade Web 2.0 without changing the way you deliver your existing IT. Please note here that again I'm not talking about WS-* v REST as those are just implementation technology decisions. What I'm talking about is creating an existing IT estate that can be easily consumed by "Mashup" or dynamic applications.
The key here is that a lot of future value for organisations is going to be based around that form of external collaboration with suppliers, customers and partners. Enabling that is a goal of both SOA (thinking), SOA (technology), Web 2.0 (technology) and Web 2.0 (Kool-aid drinking PPT jockeys). This won't be done in a green field environment it will be done based on the existing applications, ERPs and mainframes that the organisation has today and which were never intended to work in that way.
SOA doesn't need Web 2.0, but its looking like being the best interaction model for future systems.
Web 2.0 does need SOA if its going to help enterprises deliver external value.
AJAX + SOA: The Next Killer App
SOA lacks a face; that's where AJAX comes in - it puts a face on SOA
Enterprises trying to improve business unit productivity and the reuse of IT assets continue to struggle. IT organizations have achieved some success by attacking these challenges with Service Oriented Architecture (SOA), but in most cases have still only exposed small portions of the overall IT service portfolio.
The fact is that SOA is middleware - and middleware traditionally relies on more middleware to translate data into a consumer-friendly state. It's certainly a major disappointment when you finally get your SOA right only to find that building a composite application requires using a portal (middleware) and/or orchestrating it with a BPEL engine (even more middleware.
AJAX is driving a renewed interest in SOA, especially in the mashup space. But how can two very different technologies combine and connect to provide something far greater than the part
The ability to apply logic in the client (browser) and to access server data without disrupting the Web page - are what al-low the new Web 2.0 paradigm to open up so many enticing possibilities for rich enterprise applications.
Earlier I said that SOA was lacking a face. That's where AJAX comes in - it puts a face on SOA.
I see this as an opportunity to begin to apply AJAX + SOA to drive a whole new class of Web 2.0 business applications.
What I really want is a user-based composite application, not a middleware-based composite application. I truly believe we're entering a new era with amazing opportunity. Web 2.0 social networks, photo-sharing and tagging are great, but the real corporate impact comes in a form of Web 2.0 for the enterprise.