"MyMethod can’t return an object of with the type name MyMethodResult."You're fracking kidding me right? Wow. (and there are more along these lines)
http://lukesh.wordpress.com/2008/11/24/very-important-limitations-of-flex-data-services/
After fighting with the above and other bugs I was rewriting a lot of the generated code from FlexBuilder and it was just pointless. And sure, generated code isn't the greatest to rely on anyway, but give me a break. In the end I used the WebORB presentation server to handle the communication to our .NET code, as well as for generation of the initial proxy classes for the client and I have to say it was an excellent experience compared to the crap built into FlexBuilder.