I want to build a highly scalable "facebook for business" and after much testing am pretty set on FlashBuilder4 (Flex) with Google App Engine (GAE).
What are your thoughts on the need/advisability of also using Spring/Cairngorm/etc frameworks?
Frankly I don't have extensive experience coding, however feel the "MVC utopia" universally aspired to becomes convoluting when duplicating between front and back ends as envisioned above (i.e. Flex/ActionScript and GAE/Java respectively).
The purpose of using a framework is that the long-term maintenance of your application is more easily done because you have coded consistently with specific standards. You can also have multiple developers working in parallel and easily "piece" the parts back together if they are done consistently.
At least that is part of the promise frameworks provide, in theory.
A framework is built to solve a specific problem in development. Before choosing a framework, you should review your application and decide if a framework will help you solve those problems. If it can great; if it can't you shouldn't force your app to use a framework.
If you are duplicating a lot of code between the front and back end, that seems problematic to me. I'm not surprised that value objects would be mirrored between the front and back end, but their whole purpose is transferring data between different systems. Other code, or business logic, should ideally only exist in one system.
As an aside; What an oddly phrased question. If you read specific blogs of experts and want their opinions, why are you posting here instead of contacting them directly?
The combination of Flex and GAE can be very powerful. However GAE does have limitations that may impact what you are trying to build. For instance GAE doesn't support Spring last time I checked. Another cloud alternative that may work better is the new partnership between VMWare (SpringSource) and Salesforce.com called VMforce.