Stuff

                        <a4j:commandButton value="Add" status="waitStatus"
                                           action="#{UserDataController.createUserAttribute()}"
                                           render="edit_user_modal_panel"                                           
                                           oncomplete="#{rich:component('edit_user_modal_panel')}.show()"/>

<a4j:commandButton value="Add" status="waitStatus"
                                           data="#{UserDataController.createUserAttribute()}"
                                           reRender="edit_user_modal_panel"
                                           ignoreDupResponses="false"
                                           oncomplete="Richfaces.showModalPanel('edit_user_modal_panel');"/>
                    </h:panelGrid>