Currently, Event Queue with session or application scope will start a thread for handling the server push (activate/deactivate). It might cause problems when using in a clustering environment. Technically, it is possible if we 'use' the servlet thread that receives client-polling or comet request.
Ready since Nov 22, 2010
spec:
http://books.zkoss.org/wiki/ZK_Developer%27s_Reference/Server_Push/Asynchronous_Tasks