<a4j:outputPanel ajaxRendered="true" id="panelBotones">
<a4j:commandButton id="botonModificar" value="${msg.boton_modificar}"
action="#{contactosPaginadorBean.cargarContacto}" reRender="contactosRiFa"
onclick="Richfaces.showModalPanel('mpAnadirContactos',{width:325, top:'auto'})"
title="${msg.boton_modificar}" styleClass="agbotonbotonera" disabled="#{empty contactosPaginadorBean.modeloContactos.id}" />
<a4j:commandButton id="botonEliminar" value="${msg.boton_eliminar}"
action="#{contactosPaginadorBean.eliminarContacto}" reRender="contactosRiFa"
title="${msg.boton_eliminar}" styleClass="agbotonbotonera" disabled="#{empty contactosPaginadorBean.modeloContactos.id}" />
</a4j:outputPanel>
Do you have <h:form> around? If not, put your buttons in a form.