<h:form>
  <ace:simpleSelectOneMenu id="countryIn" value="#{dataBean.country}" required="true">
      <f:selectItem itemValue="" itemLabel="--Select--" noSelectionOption="true"/>
      <f:selectItem itemValue="1" itemLabel="Canada"/>
  </ace:simpleSelectOneMenu>
  <h:message for="countryIn"/>
  <h:commandButton value="Submit" actionListener="#{formBean.submit}"/>
</h:form>

When an f:selectItem with noSelectionOption="true" is used as a default value for an ace:simpleSelectOneMenu (with required="true"), if the user selects it and submits the form the field isn't considered invalid. So the noSelectionOption doesn't appear to work with ace:simpleSelectOneMenu.
This behaviour should be the same as h:selectOneMenu, where we would see the standard validation message ("Required value...").

Other selection components that use SelectItem's, like ace:selectMenu and ace:comboBox, should also support the noSelectionOption attribute, if appropriate.