Ok, I just realize it's also about the order in the markup file.
http://code.google.com/p/wiquery-plugins/source/browse/trunk/wiquery-plugins/other-plugins/src/main/java/org/odlabs/wiquery/plugin/layout/Layout.html:
<body>
<wicket:panel>
<div class="ui-layout-center"
wicket:id="paneCenter">
<span wicket:id="layoutCenter"></span>
</div>
<div class="ui-layout-north"
wicket:id="paneNorth">
<span wicket:id="layoutNorth"></span>
</div>
<div class="ui-layout-south"
wicket:id="paneSouth">
<span wicket:id="layoutSouth"></span>
</div>
<div class="ui-layout-east"
wicket:id="paneEast">
<span wicket:id="layoutEast"></span>
</div>
<div class="ui-layout-west"
wicket:id="paneWest">
<span wicket:id="layoutWest"></span>
</div>
</wicket:panel>
</body>
..and I just wanted to add, that if I am wrong, please tell me. Until
now, I am just in the very beginning of a process called learning :-)