I have the same problem:
at runtime when I try to call request.fire().
I tried including in my WEB-INF/lib several combinations of these jars (gwt-servlet.jar, gwt-servlet-deps.jar, requestfactory-servlet.jar, validation-api-1.0.0.GA.jar), without success.
I only realized that validation-api-1.0.0.GA.jar is needed in my application (otherwise the RF Servlet does not respond properly on /gwtRequest) and that JSONException is actually included in gwt-servlet-deps.jar and requestfactory-servlet.jar.
Any help? Thanks,