It means your class com.example.chat.shared.Message is not serializable, and thus can't be sent by RPC.
See the docs: http://code.google.com/intl/en/webtoolkit/doc/latest/DevGuideServerCommunication.html#DevGuideSerializableTypes ~O -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To post to this group, send email to google-appeng...@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
