I was reading the portlet section of the ICEfaces Developer Guide and came across this text:

By default, AJAX Push is active in ICEfaces. The configuration parameter that controls this is
com.icesoft.faces.synchronousUpdate. If you don't need server-initiated rendering, then you
should set this parameter to false by adding the following to your web.xml file:
<context-param>
  <param-name>com.icesoft.faces.synchronousUpdate</param-name>
  <param-value>false</param-value>
</context-param>
If you are using AJAX Push then you can set the parameter to true or just leave it out altogether.

BUT ELSEWHERE in the documentation, it says this:

If not specified, the parameter value is false; thus, by default, the application will run in asynchronous mode.

So I think that the portlet section has the wrong understanding -- when I set this to TRUE in the sample-icefaces-ipc-ajax-push-portlet attached to ICE-2896 the portlet didn't really work at all, because Asynchronous update was disabled.