Case B solution was outlined in my next post: http://www.roussev.org/2010/02/gwt-dynamic-exception-handling/ where I am demonstrating how to propagate exceptions to client without having to declare them anywhere. Probably you have more elegant solution? At that time I wrote the code there were no information so I had to extend GWT RemoteServiceServlet to achieve more dynamic approach.

I was saying : does not implement the IsSerializable interface (in older versions of GWT) or the Serializable interface (in more recent versions).

in fact, it is possible to make work case B. I have this exact mechanism used successfully in one GWT app. The exception you got for this case means that your exception class is not in one of the client package or does not implement the IsSerializable interface (in GWT 2.0).

I encountered the exact same problem, and all this is explained in the doc, but at the time, I found it difficult to find the information.