See forum thread for complete details. Basically, certain Spring JSP tags (spring:eval in particular) expect the ConversionService instance to be exposed on the pageContext/request, otherwise they create a new default instance. This instance does not include custom converters for Roo-generated entities - leading to ConverterNotFoundExceptions.
The problem is only evident when accessing views with the spring:eval tag using WebFlow, since the <mvc:annotation-driven> tag automatically registers the appropriate ConversionServiceExposingInterceptor for MVC @Controller mappings. There is no such automatic registration for the FlowHandlerMapping.
I've attached a fragment of the changes to Roo-generated webflow-config.xml that accomplish this. Note this fragment includes two additional edits: the commenting out of the AnnotationMethodHandlerAdapter (also automatically registered by mvc:annotation-driven) and SimpleControllerHandlerAdapter (unnecessary for Roo since Roo uses @Controller instances by default). These last 2 edits perhaps need their own issue.